Ion channel assay methods

ABSTRACT

A method of characterizing the biological activity of a candidate compound may include exposing cells to the candidate compound, and then exposing the cells to a repetitive application of electric fields so as to set the transmembrane potential to a level corresponding to a pre-selected voltage dependent state of a target ion channel.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority under 35 U.S.C. Section 119(e)to U.S. Provisional Patent Application No. 60/217,671, entitled“INSTRUMENTATION AND METHODS FOR ELECTRICAL STIMULATION”, filed on Jul.10, 2000, which application is hereby incorporated by reference in itsentirety.

[0002] This application is a continuation-in-part of, and hereby claimspriority to and incorporates by reference in its entirety under 35U.S.C. §120, co-pending U.S. patent application Ser. No. 09/804,457,entitled “ION CHANNEL ASSAY METHODS”, filed Mar. 12, 2001, attorneydocket AUROBIO.026A.

[0003] This application is also related to the following threeadditional co-pending U.S. Patent Applications, they are alsoincorporated by reference to this application in their entireties:

[0004] U.S. patent application Ser. No. 09/804,480, entitled “IONCHANNEL ASSAY METHODS”, filed Mar. 12, 2001, attorney docketAUROBIO.026DV1;

[0005] U.S. patent application Ser. No. 09/804,580, entitled “HIGHTHROUGHPUT METHOD AND SYSTEM FOR SCREENING CANDIDATE COMPOUNDS FORACTIVITY AGAINST TARGET ION CHANNELS”, filed Mar. 12, 2001, attorneydocket AUROBIO.026DV2; and

[0006] U.S. patent application Ser. No. 09/804,458, entitled “MULTI-WELLPLATE AND ELECTRODE ASSEMBLIES FOR ION CHANNEL ASSAYS”, filed Mar. 12,2001, attorney docket AUROBIO.026DV3.

BACKGROUND OF THE INVENTION

[0007] 1. Field of the Invention

[0008] The present invention relates generally to instrumentation andmethods for manipulating membrane potentials of living cells viaelectrical stimulation.

[0009] 2. Description of the Related Art

[0010] It has long been known that the interior of animal and plantcells is electrically negative with respect to the exterior. Themagnitude of this potential difference is generally between 5 and 90 mV,with most of the potential being developed across the cell membrane. Thetransmembrane potential of a given cell is set by the balance of theactivities of ion transporters and channels which create and maintainthe electrochemical gradient, and the activities of ion channels,passive diffusion and other factors, that allow ions to flow across theplasma membrane.

[0011] Ion channels participate in, and regulate, cellular processes asdiverse as the generation and timing of action potentials, energyproduction, synaptic transmission, secretion of hormones and thecontraction of muscles, etc. Many drugs exert their specific effects viamodulation of ion channels. Examples include antiepileptic compoundslike phenytoin and lamotrigine, which block voltage-dependent sodiumchannels in the brain, antihypertensive drugs like nifedipine anddiltiazem, which block voltage-dependent calcium channels in smoothmuscle cells, and stimulators of insulin release like glibenclamide andtolbutamide, which block ATP-regulated potassium channels in thepancreas.

[0012] Finding new drugs which have specific modulatory effects on ionchannels requires methods for measuring and manipulating the membranepotential of cells with the ion channels present in the membrane. Anumber of methods exist today that can be used to measure celltransmembrane potentials and to measure the activities of specific ionchannels. Probably the best known approach is the patch clamp,originally developed by Neher, Sakmann, and Steinback. (TheExtracellular Patch Clamp, A Method For Resolving Currents ThroughIndividual Open Channels In Biological Membranes”, Pfluegers Arch. 375;219-278, 1978). Other methods include optical recording ofvoltage-sensitive dyes (Cohen et al., Annual Reviews of Neuroscience 1:171-82, 1978) and extracellular recording of fast events using metal(Thomas et al., Exp. Cell Res. 74: 61-66, 1972) or field effecttransistors (FET) (Fromherz et al., Science 252: 1290-1293, 1991)electrodes.

[0013] The patch clamp technique allows measurement of ion flow throughion channel proteins and the analysis of the effect of drugs on ionchannels function. In brief, in the standard patch clamp technique, athin glass pipette is heated and pulled until it breaks, forming a verythin (<1 μm in diameter) opening at the tip. The pipette is filled withsalt solution approximating the intracellular ionic composition of thecell. A metal electrode is inserted into the large end of the pipette,and connected to associated electronics. The tip of the patch pipette ispressed against the surface of the cell membrane. The pipette tip sealstightly to the cell and isolates a few ion channel proteins in a tinypatch of membrane. The activity of these channels can be measuredelectrically (single channel recording) or, alternatively, the patch canbe ruptured allowing measurements of the combined channel activity ofthe entire cell membrane (whole cell recording).

[0014] During both single channel recording and whole-cell recording,the activity of individual channel subtypes can be further resolved byimposing a “voltage clamp” across the membrane. Through the use of afeedback loop, the “voltage clamp” imposes a user-specified potentialdifference across the membrane, allowing measurement of the voltage,ion, and time dependencies of various ion channel currents. Thesemethods allow resolution of discrete ion channel subtypes.

[0015] A major limitation of the patch clamp technique as a generalmethod in pharmacological screening is its low throughput. Typically, asingle, highly trained operator can test fewer than ten compounds perday using the patch clamp technique. Furthermore the technique is noteasily amenable to automation, and produces complex results that requireextensive analysis by skilled electrophysiologists. By comparison, theuse of optical detection systems provides for significantly greaterthroughput for screening applications (currently, up to 100,000compounds per day), while at the same time providing for highlysensitive analysis of transmembrane potential. Methods for the opticalsensing of membrane potential are typically based on translocation,redistribution, orientation changes, or shifts in spectra offluorescent, luminescent, or absorption dyes in response to the cellularmembrane potential (see generally González, et al., Drug Discovery Today4:431-439, 1999).

[0016] A preferred optical method of analysis has been previouslydescribed (González and Tsien, Chemistry and Biology 4: 269-277, 1997;González and Tsien, Biophysical Journal 69: 1272-1280, 1995; and U.S.Pat. No. 5,661,035 issued Aug. 26, 1997, hereby incorporated byreference). This approach typically comprises two reagents that undergoenergy transfer to provide a ratiometric fluorescent readout that isdependent upon the membrane potential. The ratiometric readout providesimportant advantages for drug screening including improved sensitivity,reliability and reduction of many types of experimental artifacts.

[0017] Compared to the use of a patch clamp, optical methods of analysisdo not inherently provide the ability to regulate, or clamp, thetransmembrane potential of a cell. Clamping methods are highly desirablebecause they provide for significantly enhanced, and more flexiblemethods of ion channel measurement. A need thus exists for reliable andspecific methods of regulating the membrane potentials of living cellsthat are compatible with optical methods of analysis and are readilyamendable to high throughput analysis.

SUMMARY OF THE INVENTION

[0018] In some embodiments, a method of characterizing the biologicalactivity of a candidate compound includes: placing a population of cellsinto an area of observation in a sample well; exposing the population ofcells to the compound; exposing the population of cells to electricfields to produce a controlled change in transmembrane potential of thepopulation of cells, wherein the electric fields include a first pulseseries and a second pulse series with a pause between the first pulseseries and the second pulse series; and monitoring changes in thetransmembrane potential of the population of cells during at least aportion of the first pulse series and a portion of the second pulseseries. In some advantageous embodiments, the monitoring includesoptically monitoring, such as by detecting fluorescence emission of aFRET based voltage sensor from an area of observation containing thepopulation of cells. Some advantageous embodiments, further includecomparing data gathered from the first pulse series with data gatheredfrom the second pulse series. In some advantageous embodiments, thechanges in the transmembrane potential are indicative of ion channelrecovery from block by the compound. In some embodiments, the pause hasa duration that is at least as long as twice the time interval betweenany two pulses in the first pulse series.

[0019] In other embodiments, a method of characterizing the biologicalactivity of a candidate compound includes: placing a population of cellsinto an area of observation in a sample well; exposing the population ofcells to the compound; exposing the population of cells to electricfields to produce a controlled change in transmembrane potential of thepopulation of cells, wherein the electric fields include a series ofpulses; monitoring changes in the transmembrane potential of thepopulation of cells; and comparing concentration dependence oftransmembrane potential response to the electric fields at a firstportion of the series of pulses with concentration dependence oftransmembrane potential response to the electric fields at a secondportion of the series of pulses. In some advantageous embodiments, themonitoring includes optically monitoring, such as by detectingfluorescence emission of a FRET based voltage sensor from an area ofobservation containing the population of cells. In some advantageousembodiments, the changes in the transmembrane potential are indicativeof use dependent block of an ion channel in the population of cells.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 Shows one embodiment of a dipper electrode array.

[0021]FIG. 2 Shows a number of embodiments of multiwell platescomprising surface electrodes.

[0022]FIG. 3 Shows a block diagram of one embodiment of the electricalstimulation system.

[0023]FIG. 4. Shows the simulated effects of repetitive externalelectrical fields on a cell expressing a voltage dependent sodiumchannel. The upper panel indicates the applied electrical field, themiddle panel indicates the simulated sodium current into the cell, andthe lower panel indicates the simulated average transmembrane potential.

[0024]FIG. 5 Shows a schematic representation of a square wave.

[0025]FIG. 6 Shows examples of various wave kernels.

[0026]FIG. 7 Shows calculated electric field profiles for variouselectrode assemblies in round, 6.2 mm diameter wells. Dashed circle is a3 mm diameter view window. In white areas, the electric field strengthis less than 10% of the average electric field strength in the viewwindow. In gray areas, the electric field strength is within 10% of theaverage electric field strength in the view window. In black areas, theelectric field strength is greater than 10% of the average electricfield strength in the view window.

[0027]FIG. 8 Shows calculated electric field profiles for variouselectrode assemblies in round and square wells 6.2 mm across. Dashedcircle is a 3 mm diameter view window. In white areas, the electricfield strength is less than 1% of the average electric field strength inthe view window. In gray areas, the electric field strength is within 1%of the average electric field strength in the view window. In blackareas, the electric field strength is greater than 1% of the averageelectric field strength in the view window.

[0028]FIG. 9 Shows various electrode and insulator designs for improvingelectric field uniformity in round wells.

[0029]FIG. 10 Shows the effect of electrical stimulation protocols atvarying pulse amplitudes over the time course of electrical stimulationin wild-type CHO cells.

[0030]FIG. 11 Shows the relationship between the maximal cellularresponse and the applied pulse amplitude during electrical stimulationfor wild-type CHO cells. Data was from FIG. 10 taken after about 5seconds.

[0031]FIG. 12 Shows the dose response curve for the effect of TTX inwild-type CHO cells. Stimulation parameters were 33 V/cm, 50 Hz for 3seconds with a biphasic square wave kernel (5 ms per phase). The solidline is a Hill function fit to the data with EC₅₀=9 nM and a Hillcoefficient of 1.47.

[0032]FIG. 13 Shows the relationship between pulse duration andfrequency and the cellular response wild-type CHO cells duringelectrical stimulation. The electric field strength was always 25 V/cm.The stimulus was a three-second burst of biphasic pulses of varyingduration and frequency. Solid lines are fits to the form$R = {1 + {\frac{A\quad f}{f + f_{0}}.}}$

[0033]FIG. 14 Shows time traces for CHO cells expressing the NaV2 sodiumchannel cells electrically stimulated at various field strengths. Cellswere stimulated in a 96-well plate, with a 20 Hz, 3 second-long train ofbiphasic, 5 ms/phase voltage pulses. The stimulation occurred during theshaded portion of the graph. In this experiment, the cells were stainedwith 20 μM CC2-DMPE and 63 nM DiSBAC₆(3). This dye combination has a 2ms time constant and accurately tracks the transmembrane potential. Therise and fall times of the response were fitted to exponential decayfunctions and were found to be τ_(rise)=200 ms and τ_(fall)=850 ms.

[0034]FIG. 15 Shows the relationship between the electric field strengthand the cellular response measured after 4 seconds (squares) and 10seconds (circles) of electrical stimulation. The line is a Boltzman fitto the data.

[0035]FIG. 16 Shows the effect of pulse duration and stimulationfrequency on the cellular response of CHO cells expressing the NaV2sodium channel.

[0036]FIG. 17 Shows the knee time parameter T₀ from the fits to the datain FIG. 16 plotted versus the stimulus duration.

[0037]FIG. 18 Shows the temporal response of HEK-293 cells expressingthe NaV3 sodium channel during electrical stimulation.

[0038]FIG. 19 Shows dose response curves for tetracaine (FIG. 19A) andtetrodotoxin (FIG. 19B) for HEK-293 expressing the NaV3 sodium channel.Electrical stimulation conditions were: E=33 V/cm, 10 ms/phase biphasicstimulation, 15 Hz burst for 1.5 seconds.

[0039]FIG. 20 Shows a dose response curve for tetracaine for HEK-293expressing the NaV4 ion channel. For this experiment, electricalstimulation parameters were E=33 V/cm, 10 ms/phase biphasic stimulation,15 Hz burst for 1.5 seconds.

[0040]FIG. 21. Shows a full-plate view of electrical stimulation ofwild-type HEK-293 cells. Each individual panel represents the time traceof the normalized fluorescence ratio of a single well in the 96-wellplate. Each well in a vertical column was stimulated simultaneously withthe same field strength. Field strength increases from left to right.Rows 6-8 contained 10 mM TEA to block the voltage-dependent potassiumchannels.

[0041]FIG. 22. Shows the cellular response as a function of the stimulusfield for wild type HEK. Error bars are standard deviations. Opensymbols: no added blockers. Filled symbols: 10 mM TEA added to blockpotassium channels.

[0042]FIG. 23 Shows the time response traces for selected concentrationsof the sodium channel blockers tetrodoxin (TTX) (FIG. 23A) andtetracaine (FIG. 23B) in CHO cells expressing the NaV2 sodium channel.

[0043]FIG. 24 Shows the dose response curves for TTX and tetracaineinhibition of the NaV2 sodium channel.

[0044]FIG. 25. Shows a ‘Random’ TTX spiking experiment. Each small boxin this 11×8 array contains the ten-second time trace of a well at thecorresponding position of a 96-well plate. The twelfth column was acontrol well without cells used for background subtraction and is notshown. Wells (1,1), (2,2), (3,3), etc. contained a blockingconcentration of TTX.

[0045]FIG. 26 Shows an analysis of the ‘random’ TTX spiking data shownin FIG. 25. The data points are the ratiometric response in the timewindow from 1.8-2.4 seconds after the beginning of the stimulus burst(i.e. at the peak of the response). The filled circles points werespiked with 1 μM TTX; the open circles had no blocker added.

[0046]FIG. 27. Shows a full-plate view of electrically-stimulated HL5cardiac muscle cells. Each individual panel represents the time trace ofthe normalized fluorescence ratio of a single well in the 96-well plate.Each well in a vertical column was stimulated simultaneously with thesame field strength. Field strength increases from left to right. Rows 5and 6 contained 10 μM TTX to partially block the voltage-dependentsodium channels. Rows 7 and 8 contained 10 mM TEA to partially block thevoltage-dependent potassium channels.

[0047]FIG. 28. Shows the response of HL5 cells as a function of theapplied electric field strength. Black points are the average of theresponse of four wells with no added compounds. The solid line is aBoltzman fit to the data with E₅₀=22 V/cm. The points are the screeningwindow: the difference between the response and the unstimulatedresponse normalized to the standard deviation of the response (seeAppendix A3).

[0048]FIG. 29 The typical voltage response for CHO cells expressing apotassium channel and the NaV3 sodium channel after a three separatestimulation cycles using surface electrodes.

[0049]FIG. 30 Shows the average ratiometric response of a population ofcells grown in a 96 well multiwell plate stimulated with monophasicstimuli of varying field strengths via surface electrodes. The points inthis curve are the average peak response of 4 stimulations on the sameculture.

[0050]FIG. 31 Shows the cellular response as a function of the stimulusfield for wild type RBL. Error bars are standard deviations. Opensymbols: no added blockers. Filled symbols: 400 μM TEA added to blockIRK1 channels.

[0051]FIG. 32 Shows the use-dependent effects of different compounds atvarying concentrations using repetitive electrical stimulation at 1 Hzfor 20 seconds. FIG. 32A shows the results of four compounds while FIG.32B superimposes tetracaine and TTX to illustrate the distinctionbetween a use-dependent and non-use-dependent compound.

[0052]FIG. 33 Shows the use dependent effects of four differentcompounds using repetitive electrical stimulation at 1 Hz for 20 secondsand then comparing the 1^(st) and 20^(th) peak responses.

[0053]FIG. 34A Shows an example of a stimulation protocol to studyrecovery of channels from tonic block and use-dependent block. FIG. 34Bshows concentration dependent recovery from tetracaine and lamotrigineblock at 0.5 s train interval.

[0054]FIG. 35 Shows the concentration dependence of the recovery ofchannels from lamotrigine block and tetracaine block.

[0055]FIG. 36A Shows the results for lamotrigine block at 50 μM fordifferent time intervals while FIG. 36B shows the results for tetracaineblock at 4 μM for different time intervals.

[0056]FIG. 37 Shows the recovery time contrast between tetracaine andlamotrigine.

[0057]FIG. 38 Shows reduction of firing frequency by bupivacaine andlidocaine in Aδ- or C-type neuron slice preparation.

[0058]FIG. 39 Shows that induced spontaneous firing of Nav1.8 expressingcells is slowed by lamotrigine and lidocaine.

[0059]FIG. 40 Shows detection of the use-dependence of Mibefradil usinga hexyl assay.

[0060]FIG. 41 Shows the effect of increased potassium levels on sodiumchannel blockage by lamotrigine and tetracaine.

[0061]FIG. 42 Shows block of the L-type calcium channel by nifedipineand by nimodipine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0062] Generally, the nomenclature used herein and many of thefluorescence, computer, detection, chemistry and laboratory proceduresdescribed below are those well known and commonly employed in the art.Standard techniques are usually used for chemical synthesis,fluorescence, optics, molecular biology, computer software andintegration. Generally, chemical reactions, cell assays and enzymaticreactions are performed according to the manufacturer's specificationswhere appropriate. The techniques and procedures are generally performedaccording to conventional methods in the art and various generalreferences, including those listed below, which are herein incorporatedby reference.

[0063] Lakowicz, J. R. Topics in Fluorescence Spectroscopy, (3 volumes)New York: Plenum Press (1991), and Lakowicz, J. R. Emerging applicationsof fluorescence spectroscopy to cellular imaging: lifetime imaging,metal-ligand probes, multi-photon excitation and light quenching.Scanning Microsc Suppl Vol. 10 (1996) pages 213-24, for fluorescencetechniques;

[0064] Sambrook et al. Molecular Cloning: A Laboratory Manual, 2^(nd)ed. (1989) Cold Spring Harbor Laboratory Press, Cold Spring Harbor,N.Y., for molecular biology methods;

[0065]Cells: A Laboratory Manual, 1^(st) edition (1998) Cold SpringHarbor Laboratory Press, Cold Spring Harbor, N.Y., for cell biologymethods;

[0066]Optics Guide 5 Melles Griot® Irvine Calif., Optical WaveguideTheory, Snyder & Love published by Chapman & Hall for general opticalmethods;

[0067] Hille, B. Ionic Channels of Excitable membranes, Second Edition(1992) Sinauer Associates, Inc., Sunderland, Mass. for generalelectrophysiological methods and properties of ion channels.

[0068] Horowitz and Hill, The Art of Electronics, Second Edition (1989)Cambridge University Press, Cambridge, U.K. for electronic circuits.

[0069] The following definitions are set forth to illustrate and definethe meaning and scope of the various terms used to describe theinvention herein.

[0070] The term “activation” refers to the transition from a resting(non-conducting) state of an ion channel to the activated (conducting)state.

[0071] The term “activation threshold” refers to the lowest potentialabove which measurable opening of a channel occurs.

[0072] The term “anode” refers to an electrode when driven to a positivepotential relative to earth by an external source.

[0073] The term “area of cellular stimulation” means the area defined bytwo electrodes that experiences significant electrical stimulation(typically 5V/cm or higher) in which the cells of interest are located.Typically the area of cellular stimulation is larger than, or equal to,the area of observation. For standard 96-well based measurements thearea of cellular stimulation is typically about 16 mm².

[0074] The term “area of observation” means the portion of the systemover which a measurement is taken. The area of observation is typicallyan area of at least 0.5 mm² for multiwell plate based measurements.

[0075] The term “bioluminescent protein” refers to a protein capable ofcausing the emission of light through the catalysis of a chemicalreaction. The term includes proteins that catalyze bioluminescent orchemiluminescent reactions, such as those causing the oxidation ofluciferins. The term “bioluminescent protein” includes not onlybioluminescent proteins that occur naturally, but also mutants thatexhibit altered spectral or physical properties.

[0076] The term “biphasic” refers to a pulse with two parts, each withan opposite polarity.

[0077] The term “Boltzman function” refers to the sigmoidal (i.e.step-like) response function${y(x)} = {y_{0} + {\frac{A}{1 + {\exp \left( \frac{x - x_{50}}{\Delta \quad x} \right)}}.}}$

[0078] Where: y is the independent variable

[0079] y₀ is an adjustable parameter equal to the limit of the functionas x→∞

[0080] A is an adjustable parameter equal to step size

[0081] x₅₀ is an adjustable parameter related to the midpoint of thestep

[0082] Δx is an adjustable parameter describing the width of the step

[0083] The term “cathode” refers to an electrode when driven to anegative potential relative to earth by an external source.

[0084] The term “depolarize” means to cause the transmembrane potentialof a cell to become closer to zero. In the case of cells that arenormally at negative resting potentials, this term means that thetransmembrane potential changes in a positive direction.

[0085] The term “effective concentration (50%)” or “EC₅₀” refers to theconcentration at which a pharmacological compound has half theeffectiveness compared to the maximal effectiveness at highconcentrations of the compound.

[0086] The term “electrically excitable” refers to a cell or tissue thatresponds to a suprathreshold electrical stimulus by generating an actionpotential. Electrically excitable cells contain at least onevoltage-dependent ion channel type generating an inward current and atleast one ion channel type generating an outward current.

[0087] The term “electrical stimulation” means initiating a voltagechange in cells using an extracellular current pulse.

[0088] The term “electrode” means a controllable conductive interfacebetween an instrument and a test system.

[0089] The term “electropermeablization” refers to mild electroporation,in which the hydrated pores created through the membrane are only largeenough to pass water molecules and small single-atom ions.

[0090] The term “electroporation” refers to a phenomenon in which theapplication of a large electric potential across the membrane of a cellresults in dielectric breakdown of the membrane, and the creation ofhydrated pathways through the membrane.

[0091] The term “fluorescent component” refers to a component capable ofabsorbing light and then re-emitting at least some fraction of thatenergy as light over time. The term includes discrete compounds,molecules, naturally fluorescent proteins and marco-molecular complexesor mixtures of fluorescent and non-fluorescent compounds or molecules.The term “fluorescent component” also includes components that exhibitlong lived fluorescence decay such as lanthanide ions and lanthanidecomplexes with organic ligand sensitizers, that absorb light and thenre-emit the energy over milliseconds.

[0092] The term “FRET” refers to fluorescence resonance energy transfer.For the purposes of this invention, FRET includes energy transferprocesses that occur between two fluorescent components, a fluorescentcomponent and a non-fluorescent component, a luminescent component and afluorescent component and a luminescent component with a non-fluorescentcomponent.

[0093] The term “gene knockout” as used herein, refers to the targeteddisruption of a gene in vivo with complete loss of function that hasbeen achieved by any transgenic technology familiar to those in the art.In one embodiment, transgenic animals having gene knockouts are those inwhich the target gene has been rendered nonfunctional by an insertiontargeted to the gene to be rendered non-functional by homologousrecombination.

[0094] The term “Hill function” refers to the sigmoidal (i.e. step-like)response function ${y(x)} = {y_{0} + {\frac{A}{x_{0}^{n} + x^{n}}.}}$

[0095] Where: y is the independent variable

[0096] y₀ is an adjustable parameter equal to the limit of the functionas x→∞

[0097] A is an adjustable parameter equal to step size

[0098] x₀ is an adjustable parameter related to the midpoint of the step

[0099] n is an adjustable parameter describing the steepness of the step

[0100] The term “Hill coefficient” refers to the parameter n in the Hillfunction.

[0101] The term “hit” refers to a test compound that shows desiredproperties in an assay.

[0102] The term “homolog” refers to two sequences or parts thereof, thatare greater than, or equal to 75% identical when optimally aligned usingthe ALIGN program. Homology or sequence identity refers to thefollowing. Two amino acid sequences are homologous if there is a partialor complete identity between their sequences. For example, 85% homologymeans that 85% of the amino acids are identical when the two sequencesare aligned for maximum matching. Gaps (in either of the two sequencesbeing matched) are allowed in maximizing matching; gap lengths of 5 orless are preferred with 2 or less being more preferred. Alternativelyand preferably, two protein sequences (or polypeptide sequences derivedfrom them of at least 30 amino acids in length) are homologous, as thisterm is used herein, if they have an alignment score of more than 5 (instandard deviation units) using the program ALIGN with the mutation datamatrix and a gap penalty of 6 or greater. See Dayhoff, M. O., in Atlasof Protein Sequence and Structure, 1972, volume 5, National BiomedicalResearch Foundation, pp. 101-110, and Supplement 2 to this volume, pp.1-10.

[0103] The term “hyperpolarize” means to cause the transmembranepotential of a cell to move farther away from zero. In the case of cellsthat are normally at negative resting potentials, this term means thatthe transmembrane potential changes in a negative direction.

[0104] The term “inactivation” means that an ion channel moves into theinactivated state.

[0105] The term “inactivated” refers to a voltage-dependent ion channelin a particular non-conducting conformational state. Transitions intoand out of the inactivated state are generally slow relative totransitions between other conformational states. The inactivated stateis usually the preferred state at elevated transmembrane potentials. Atlow transmembrane potentials, the inactivated state is unstable andrelaxes to the resting state.

[0106] The term “kernel” means a mathematical function intended to beconvoluted with one or more other time-varying functions: In theory, thekernel can be any function that tends to zero as the independentvariable tends to ±∞. In practice, the kernel can be any waveform thatcan programmed into an arbitrary wavefunction generator, or that can begenerated by a computer-controlled digital to analog (D/A) converter.

[0107] The term “luminescent component” refers to a component capable ofabsorbing energy, such as electrical (e.g. Electro-luminescence),chemical (e.g. chemi-luminescence) or acoustic energy and then emittingat least some fraction of that energy as light over time. The term“component” includes discrete compounds, molecules, bioluminescentproteins and macro-molecular complexes or mixtures of luminescent andnon-luminescent compounds or molecules that act to cause the emission oflight.

[0108] The term “transmembrane potential modulator” refers to componentscapable of altering the resting or stimulated transmembrane potential ofa cellular or sub-cellular compartment. The term includes discretecompounds, ion channels, receptors, pore forming proteins, or anycombination of these components.

[0109] The term “membrane time constant” or “τ_(M)” means the product ofthe membrane resistance (R_(M)) and capacitance (C_(M)).

[0110] The term “monophasic” refers to a pulse whose polarity does notchange to the opposite polarity.

[0111] The term “naturally fluorescent protein” refers to a proteincapable of forming a highly fluorescent, intrinsic chromophore eitherthrough the cyclization and oxidation of internal amino acids within theprotein or via the enzymatic addition of a fluorescent co-factor. Theterm includes wild-type fluorescent proteins and engineered mutants thatexhibit altered spectral or physical properties. The term does notinclude proteins that exhibit weak fluorescence by virtue only of thefluorescence contribution of non-modified tyrosine, tryptophan,histidine and phenylalanine groups within the protein. The term“naturally occurring” refers to a component produced by cells in theabsence of artificial genetic or other modifications of those cells.

[0112] The term “Multiwell plate” refers to a two dimensional array ofaddressable wells located on a substantially flat surface. Multiwellplates may comprise any number of discrete addressable wells, andcomprise addressable wells of any width or depth. Common examples ofmultiwell plates include 96 well plates, 384 well plates and 3456 wellNanoplates™.

[0113] The term “operably linked” refers to a juxtaposition wherein thecomponents so described are in a relationship permitting them tofunction in their intended manner. A control sequence “operably linked”to a coding sequence is ligated in such a way that expression of thecoding sequence is achieved under conditions compatible with the controlsequences.

[0114] The term “polarized cell” means a cell with an electric potentialdifference across its cell membrane.

[0115] The term “rectification” means that the conductance isnon-linear, with a preferred direction.

[0116] The term “release from inactivation” refers to the conversion ofan inactivated closed channel, to a resting closed channel that is nowcapable of opening.

[0117] The term “repetitive” means to repeat at least twice.

[0118] The term “repolarize” means to cause the transmembrane potentialof a cell to approach its resting potential.

[0119] The term “resting” or “resting state” refers to avoltage-dependent ion channel that is closed, but free frominactivation.

[0120] The term “resting potential” for a cell means the equilibriumtransmembrane potential of a cell when not subjected to externalinfluences.

[0121] The term “reversal potential” for a particular ion refers to thetransmembrane potential for which the inward and outward fluxes of thation are equal.

[0122] The term “substantially parallel” means that the distance betweenthe surfaces of two objects facing each other varies by less than 10%,preferably less than 5%, when measured at every point on the relevantsurface of each object.

[0123] The term “targetable” refers to a component that has the abilityto be localized to a specific location under certain conditions. Forexample, a protein that can exist at two or more locations that has theability to translocate to a defined site under some condition(s) istargetable to that site. Common examples include the translocation ofprotein kinase C to the plasma membrane upon cellular activation, andthe binding of SH2 domain containing proteins to phosphorylated tyrosineresidues. The term includes components that are persistently associatedwith one specific location or site, under most conditions.

[0124] The term “threshold electroporation potential” refers to theexternally applied field strength above which detectable electroporationof a living cell occurs.

[0125] The term “test compound” refers to a chemical to be tested by oneor more screening method(s) of the invention as a putative modulator. Atest compound can be any chemical, such as an inorganic chemical, anorganic chemical, a protein, a peptide, a carbohydrate, a lipid, or acombination thereof. Usually, various predetermined concentrations oftest compounds are used for screening, such as 0.01 micromolar, 1micromolar and 10 micromolar. Test compound controls can include themeasurement of a signal in the absence of the test compound orcomparison to a compound known to modulate the target.

[0126] The term “transformed” refers to a cell into which (or into anancestor of which) has been introduced, by means of recombinant nucleicacid techniques, a heterologous nucleic acid molecule.

[0127] The term “transgenic” is used to describe an organism thatincludes exogenous genetic material within all of its cells. The termincludes any organism whose genome has been altered by in vitromanipulation of the early embryo or fertilized egg or by any transgenictechnology to induce a specific gene knockout.

[0128] The term “transgene” refers any piece of DNA which is inserted byartifice into a cell, and becomes part of the genome of the organism(i.e., either stably integrated or as a stable extrachromosomal element)which develops from that cell. Such a transgene may include a gene whichis partly or entirely heterologous (i.e., foreign) to the transgenicorganism, or may represent a gene homologous to an endogenous gene ofthe organism. Included within this definition is a transgene created bythe providing of an RNA sequence that is transcribed into DNA and thenincorporated into the genome. The transgenes of the invention includeDNA sequences that encode the fluorescent or bioluminescent protein thatmay be expressed in a transgenic non-human animal.

[0129] The term “transistor-transistor logic” or “TTL” refers to anelectronic logic system in which a voltage around +5V is TRUE and avoltage around 0V is FALSE.

[0130] A “uniform electric field” means that the electric field variesby no more than 15% from the mean intensity within the area ofobservation at any one time.

[0131] The term “voltage sensor” includes FRET based voltage sensors,electrochromic transmembrane potential dyes, transmembrane potentialredistribution dyes, extracellular electrodes, field effect transistors,radioactive ions, ion sensitive fluorescent or luminescent dyes, and ionsensitive fluorescent or luminescent proteins, that are capable ofproviding an indication of the transmembrane potential.

[0132] The following terms are used to describe the sequencerelationships between two or more polynucleotides: “reference sequence”,“comparison window”, “sequence identity”, “percentage identical to asequence”, and “substantial identity”. A “reference sequence” is adefined sequence used as a basis for a sequence comparison; a referencesequence may be a subset of a larger sequence, for example, as a segmentof a full-length cDNA or gene sequence, or may comprise a complete cDNAor gene sequence. Generally, a reference sequence is at least 20nucleotides in length, frequently at least 25 nucleotides in length, andoften at least 50 nucleotides in length. Since two polynucleotides mayeach (1) comprise a sequence (i.e., a portion of the completepolynucleotide sequence) that is similar between the twopolynucleotides, and (2) may further comprise a sequence that isdivergent between the two polynucleotides, sequence comparisons betweentwo (or more) polynucleotides are typically performed by comparingsequences of the two polynucleotides over a “comparison window” toidentify and compare local regions of sequence similarity. A “comparisonwindow”, as used herein, refers to a conceptual segment of at least 20contiguous nucleotide positions wherein a polynucleotide sequence may becompared to a reference sequence of at least 20 contiguous nucleotidesand wherein the portion of the polynucleotide sequence in the comparisonwindow may comprise additions or deletions (i.e., gaps) of 20 percent orless as compared to the reference sequence (which does not compriseadditions or deletions) for optimal alignment of the two sequences.Optimal alignment of sequences for aligning a comparison window may beconducted by the local homology algorithm of Smith and Waterman (1981)Adv. Appl. Math. 2: 482, by the homology alignment algorithm ofNeedleman and Wunsch (1970) J. Mol. Biol. 48: 443, by the search forsimilarity method of Pearson and Lipman (1988) Proc. Natl. Acad. Sci.(USA) 85: 2444, by computerized implementations of these algorithms(GAP, BESTFIT, FASTA, and TFASTA in the Wisconsin Genetics SoftwarePackage Release 7.0, Genetics Computer Group, 575 Science Dr., Madison,Wis.), or by inspection, and the best alignment (i.e., resulting in thehighest percentage of homology over the comparison window) generated bythe various methods is selected. The term “sequence identity” means thattwo polynucleotide sequences are identical (i.e., on anucleotide-by-nucleotide basis) over the window of comparison. The term“percentage identical to a sequence” is calculated by comparing twooptimally aligned sequences over the window of comparison, determiningthe number of positions at which the identical nucleic acid base (e.g.,A, T, C, G, U, or I) occurs in both sequences to yield the number ofmatched positions, dividing the number of matched positions by the totalnumber of positions in the window of comparison (i.e., the window size),and multiplying the result by 100 to yield the percentage of sequenceidentity. The terms “substantial identity” as used herein denotes acharacteristic of a polynucleotide sequence, wherein the polynucleotidecomprises a sequence that has at least 30 percent sequence identity,preferably at least 50 to 60 percent sequence identity, more usually atleast 60 percent sequence identity as compared to a reference sequenceover a comparison window of at least 20 nucleotide positions, frequentlyover a window of at least 25-50 nucleotides, wherein the percentage ofsequence identity is calculated by comparing the reference sequence tothe polynucleotide sequence which may include deletions or additionswhich total 20 percent or less of the reference sequence over the windowof comparison.

[0133] As applied to polypeptides, the term “substantial identity” meansthat two peptide sequences, when optimally aligned, such as by theprograms GAP or BESTFIT using default gap weights, share at least 30percent sequence identity, preferably at least 40 percent sequenceidentity, more preferably at least 50 percent sequence identity, andmost preferably at least 60 percent sequence identity. Preferably,residue positions which are not identical differ by conservative aminoacid substitutions. Conservative amino acid substitutions refer to theinterchangeability of residues having similar side chains. For example,a group of amino acids having aliphatic side chains is glycine, alanine,valine, leucine, and isoleucine; a group of amino acids havingaliphatic-hydroxyl side chains is serine and threonine; a group of aminoacids having amide-containing side chains is asparagine and glutamine; agroup of amino acids having aromatic side chains is phenylalanine,tyrosine, and tryptophan; a group of amino acids having basic sidechains is lysine, arginine, and histidine; and a group of amino acidshaving sulfur-containing side chains is cysteine and methionine.Preferred conservative amino acids substitution groups are:valine-leucine-isoleucine, phenylalanine-tyrosine, lysine-arginine,alanine-valine, glutamic-aspartic, and asparagine-glutamine.

[0134] Since the list of technical and scientific terms cannot be allencompassing, any undefined terms shall be construed to have the samemeaning as is commonly understood by one of skill in the art to whichthis invention belongs. Furthermore, the singular forms “a”, “an” and“the” include plural referents unless the context clearly dictatesotherwise. For example, reference to a “restriction enzyme” or a “highfidelity enzyme” may include mixtures of such enzymes and any otherenzymes fitting the stated criteria, or reference to the method includesreference to one or more methods for obtaining cDNA sequences which willbe known to those skilled in the art or will become known to them uponreading this specification.

[0135] 1. Introduction

[0136] The present invention recognizes for the first time that thetransmembrane potentials of intact living cells comprising at least onevoltage regulated ion channel, can be precisely modulated via theapplication of repetitive electrical stimulation pulses to the fluidbathing the cells. The present invention includes instrumentation andmethods that provide for the accurate and reliable modulation of thetransmembrane potentials of intact living cells without significantlydisrupting their native cellular integrity.

[0137] As a non-limiting introduction to the breadth of the invention,the invention includes several general and useful aspects, including:

[0138] 1) Instrumentation including electrodes, and electrode arrays forreliably generating uniform electrical fields in cultures of livingcells in aqueous solution.

[0139] 2) Multiwell plates comprising surface electrodes for highthroughput and miniaturized stimulation and analysis of ion channel orcellular activities.

[0140] 3) Systems for high throughput analysis of ion channel andcellular activities and for use in drug discovery, analysis, screeningand profiling.

[0141] 4) Methods for modulating the transmembrane potential of a livingcell via the use of repetitive electrical stimulation.

[0142] 5) Methods for screening the effects of test compounds on theactivities of voltage regulated, and non-voltage regulated ion channels,transporters and leak currents. Including determining state-dependentpharmacological activity of compounds against ion channel andtransporter proteins.

[0143] 6) Methods for profiling and selecting cells or clones based ontheir response to electrical stimulation.

[0144] 7) Methods for quantitative determination of cellular and ionchannel parameters in a high-throughput manner, and for quantificationof the pharmacological effects of compounds on those parameters.

[0145] 8) Methods for the introduction of exogenous compounds into theintracellular spaces of cells.

[0146] 9) Methods for modulating the transmembrane potential ofintracellular organelles, and for screening test compounds against ionchannels in these organelles.

[0147] 10) Methods for characterizing the physiological effect of thetransmembrane potential on the function and regulation of physiologicaland biochemical responses, including gene expression, enzyme function,protein activity and ligand binding.

[0148] 11) Methods for programming or training adaptive neuronalnetworks or bio-computers for specific functional or logical responses.

[0149] 12) Methods for providing efficient neuronal interfaces forprosthetic devices implanted into an animal, including a human.

[0150] These aspects of the invention and others described herein, canbe achieved by using the methods and instrumentation described herein.To gain a full appreciation of the scope of the invention, it will befurther recognized that various aspects of the invention can be combinedto make desirable embodiments of the invention. Such combinations resultin particularly useful and robust embodiments of the invention.

[0151] 2. Electrodes and Electrode Arrays

[0152] In one embodiment, the present invention includes electrodes, andelectrode arrays, for creating electrical fields across the area ofobservation. Typically this is achieved via the use of a pair ofelectrically conductive electrodes. An important design feature is thatthe electrode pairs create well-defined electrical fields. Preferredelectrode designs include electrode configurations that maximize theelectric field homogeneity experienced by the cells under observation.

[0153] Generating uniform electric fields over the area of observationis important for electrical stimulation for several reasons. Firstly,because the cellular response is sensitive to the magnitude of the localelectric field, non-uniform fields typically cause non-uniform responsesin different areas, leading to an increased scatter in the results.Secondly, the threshold for electropermeablization is typically only afactor of 2-5 larger than the transmembrane potentials required forelectrical stimulation membrane (see Teissie and Rols, 1993, Biophys. J.65:409-413). Thus, if the electric field is too non-uniform, it may notbe possible to stimulate all the cells in the area of observationwithout also electropermeablizing some of them.

[0154] Field uniformity over a fixed area can be described in two ways:(1) the standard deviation of the field magnitude divided by the averagefield magnitude in the area, and (2) the difference between the highestand lowest field magnitudes, normalized to the average field magnitudein the area.

[0155] a. Design of Electrodes

[0156] The simplest way to generate a uniform electric field in aconductive medium is to use two identical, flat electrodes with surfacesthat are aligned substantially parallel to each other. Generally thecloser the electrodes are to each other relative to their width in thetransverse direction, the greater the field uniformity will be. Typicalround multiwell plate wells however limit the width of electrodes. thatcan be inserted into the wells, and also introduce two other effectswhich reduce field uniformity.

[0157] The roundness of the wells provides a challenge to create auniform field pointing in one direction with two electrodes the width ofthe conductive saline between the electrodes is constantly changing.Additionally the high surface tension of water generates variations inthe height of the saline across the well when dipper electrodes areinserted. The curved surface, or meniscus, can perturb the electricfield throughout the volume of the well. The depth of 100 μL of salinein a 96-well plate is normally about 3.0 mm deep at the center and about2.9 mm deep at the edges of the well. When two stainless steel parallelplate electrodes are inserted, saline is drawn up between the electrodesand the walls of the well causing depth variations over the area ofobservation suggesting that the current paths throughout the volume ofthe saline curve around the center, generating electric fieldnon-uniformity.

[0158] In one aspect the present invention includes improved electrodedesigns, and systems for electrical stimulation that address theseissues to create substantially uniform electrical fields over the areaof observation.

[0159] In one embodiment, (FIG. 9A) the electrode pair comprises twosubstantially parallel electrodes comprising an electrical insulatorthat is attached to the pair of electrodes to restrict current flow to adefined region thereby creating a highly uniform electrical field.

[0160] In another embodiment, (FIG. 9B) the electrode pair additionallycomprises satellite electrodes to create a more uniform electricalfield.

[0161] In another embodiment, (FIG. 9D) the electrode pair issub-divided into several pieces separated by thin insulating dividers.In this case the potential applied to each electrode, expressed as afraction of the potential applied to the central most piece can beindividually tuned to maximize the field uniformity in the area ofobservation.

[0162] In another aspect, the present invention includes improvedelectrode designs (FIG. 9C) that exhibit improved field uniformity overthe area of observation via the elimination or reduction of the meniscuseffect.

[0163] In another aspect multiple electric potential sensors can befabricated into the surface or walls of the wells in a multiwell plate,or attached in arrays to the dipper electrode assembly. These sensorscan be monitored to manually or automatically adjust the individualelectrodes, so as to maximize field uniformity. This arrangement will beuseful to allow a stimulating electrode array to compensate forvariations and imperfections in the well shape, volume of saline,variations in the manufacturing process for the electrodes, damage tothe electrode assembly, etc.

[0164] b. Placement of Electrodes Within the Wells

[0165] For dipper electrodes, the ideal situation (in terms of creatinga uniform electric field) would be to have the bottoms of the electrodestouching the bottom of the well. This way, there will be no fringingfields or field non-uniformity associated with vertical current paths.For a removable structure, however, it is not desirable to require theelectrodes to make contact with the surface. Small deviations in theplate geometry can cause some electrodes to press into the surface,causing damage either to the plate, the cells, or the electrodes.Additionally, in some wells, the electrodes may not extend all the wayto the surface. For these reasons it may be desirable to design a smallgap between the bottom of the electrode and the bottom of the well.

[0166] Accordingly in one aspect the present invention includesmultiwell plates in which the area of observation in the middle of thewell is raised relative to area around the circumference of the well,where the electrodes would be placed.

[0167] The fringing fields will cause non-uniformity over aninter-electrode distance roughly equal to the gap between the bottom ofthe electrode and the bottom of the well. Therefore, this gap should bekept as small as is practical, preferably in the range of 0.1 to 0.5 mmand the area of observation should not typically include any part of thewell within this distance from the electrodes.

[0168] c. Manufacture of Electrodes

[0169] Any electrically conductive material can be used as an electrode.Preferred electrode materials have many of the following properties, (1)they do not corrode in saline, (2) they do not produce or release toxicions, (3) they are flexible and strong, (4) they are relativelyinexpensive to fabricate, (5) they are non porous, and (6) they areeasily cleaned. Preferred materials include noble metals (includinggold, platinum, and palladium), refractory metals (including titanium,tungsten, molybdenum, and iridium), corrosion-resistant alloys(including stainless steel) and carbon or other organic conductors(including graphite and polypyrrole). For many embodiments stainlesssteel provides a preferred electrode material. This material isinexpensive, easy to machine, and very inert in saline. Stainless steeloxidizes slowly to produce iron oxide when passing current in saline,but this does not appear to affect the performance of the system. Ironoxide has very low solubility in water and toxic levels of iron do notappear to be released. Additionally any iron oxide deposits can easilybe removed by soaking the electrodes in 10% nitric acid in water for twohours, then rinsing thoroughly with distilled water.

[0170] Solid copper and silver electrodes may be used for someapplications but are less preferred for routine use because they corroderapidly in saline. Gold plated copper electrodes are relatively inert,but appear to lose their gold plating during prolonged electricalstimulation.

[0171] Electrolysis products can be contained or eliminated by coatingthe surfaces of the electrodes with protective coatings, such asgelatin, polyacrilimide, or agarose gels. Another potentially usefulelectrode material is an electrochemical half-cell, such as asilver/silver chloride electrode.

[0172] d. Electrode Arrays

[0173] Dipper electrodes typically consist of one or more pairs ofelectrodes that are arranged in an array that can be retractably movedinto, and out of, one or more wells of a multiwell plate. Dipperelectrodes may be orientated into arrays that match the plate format anddensity, but can be in arrays of any configuration or orientation. Forexample for a standard 96 well plate, a number of electrodeconfigurations are possible including electrode array arrangements toselectively excite one or more columns, or rows, simultaneously.

[0174] An example of one embodiment of an electrode array of this typeis shown in FIG. 1. In this example, a 12 by 8 array of electrode pairsis formatted so as to fit into a standard 96-well multiwell plate. Inthis case the electrodes (10) are approximately 4 mm wide, 1 cm long and0.2 mm thick, and extend from a conductive comb (50) that is connectedthrough a switch to one side of the output stage of a high-powerfunction generator. The electrodes are mounted parallel to each other, 4mm apart, with a non-conductive nylon spacer (20) in between. In thiscase, the switch (330) enables one column of the 96 well plate to beselectively stimulated at a time, however any temporal, or spatial,combination of stimulation protocols is potentially possible given theappropriate configuration of switching, wiring and power functiongenerator.

[0175] The entire array of electrodes is held in correct registration bya rigid non conductive member (30) that keeps each electrode paircorrectly spaced to accurately match a standard 96 well plate layout.The non-conductive member (30) provides for the electrodes to move up ordown while precisely maintaining their registration with the multiwellplate.

[0176] To provide for correct registration of the electrode array with amultiwell plate, the electrode assembly can optionally comprise an outerborder or flange (40) that can accommodate a standard 96-well plate, andenables accurate plate registration. In some embodiments the border (40)can further include a registration notch or indentation (80) to provideunambiguous plate registration.

[0177] In a preferred embodiment (Also shown in FIG. 1A) the electrodearray further comprises means for retractably inserting the electrodearray into the wells of the multiwell plate. In one embodiment of thisconfiguration, the electrode array further comprises an upper, movablesupport member (90) to which the electrodes (10) are attached. Themovable support member (90) is able to move up or down relative to thenon-conductive member (30) by sliding on four alignment pins (70). Notshown in these figures is a spring that enables the movable supportlayer (90) to automatically return to the upper position when downwardforce is no longer applied. A spacer (60) provides the ability to lockthe movable support layer (90) and electrodes (10) in the fully lowerorientation. This device allows the electrical stimulator to be used inmanual and/or robotic screening modes.

[0178] 3. Multiwell Plates for Electrical Stimulation

[0179] The multiwell plates of the present invention are designedprimarily to provide for efficient electrical stimulation of cells whileat the same time enabling the optical analysis of transmembranepotential changes. To accomplish this conductive surface electrodes maybe orientated in, or on, the walls, bottoms or lids of the multiwellplate. In general such multiwell plates can have a footprint of anyshape or size, such as square, rectangular, circular, oblong,triangular, kidney, or other geometric or non-geometric shape. Thefootprint can have a shape that is substantially similar to thefootprint of existing multiwell plates, such as the standard 96-wellmicrotiter plate, whose footprint is approximately 85.5 mm in width by127.75 mm in length, or other sizes that represent a current or futureindustry standard (see T. Astle, Standards in Robotics andInstrumentation, J. of Biomolecular Screening, Vol. 1 pages163-168,1996). Multiwell plates of the present invention having thisfootprint can be compatible with robotics and instrumentation, such asmultiwell plate translocators and readers as they are known in the art.

[0180] Typically, wells will be arranged in two-dimensional lineararrays on the multiwell plate. However, the wells can be provided in anytype of array, such as geometric or non-geometric arrays. The multiwellplate can comprise any number of wells. Larger numbers of wells orincreased well density can also be easily accommodated using the methodsof the claimed invention. Commonly used numbers of wells include 6, 12,96, 384, 1536, 3456, and 9600.

[0181] Well volumes typically can vary depending on well depth and crosssectional area. Preferably, the well volume is between about 0.1microliters and 500 microliters. Wells can be made in any crosssectional shape (in plan view) including, square, round, hexagonal,other geometric or non-geometric shapes, and combinations (intra-welland inter-well) thereof. Preferred are square or round wells, with flatbottoms.

[0182] The walls can be chamfered (e.g. having a draft angle).Preferably, the angle is between about 1 and 10 degrees, more preferablybetween about 2 and 8 degrees, and most preferable between about 3 and 5degrees.

[0183] The wells can be placed in a configuration so that the wellcenter-to well-center distance can be between about 0.5 millimeters andabout 100 millimeters. The wells can be placed in any configuration,such as a linear-linear array, or geometric patterns, such as hexagonalpatterns. The well-to-well distance can be about 9 mm for a 96 wellplate. Smaller well-center to well-center distances are preferred forsmaller volumes.

[0184] The wells can have a depth between about 0.5 and 100 millimeters.Preferably, the well depth is between about 1 millimeter and 100millimeters, more preferably between about 2 millimeters and 50millimeters, and most preferably between about 3 millimeters and 20millimeters.

[0185] The wells can have a diameter (when the wells are circular) ormaximal diagonal distance (when the wells are not circular) betweenabout 0.2 and 100 millimeters. Preferably, the well diameter is betweenabout 0.5 and 100 millimeters, more preferably between about 1 and 50millimeters, and most preferably, between about 2 and 20 millimeters.

[0186] The multiwell plate, will generally be composed of electricallynon-conductive material and can comprise an optically opaque materialthat can interfere with the transmission of radiation, such as light,through the wall of a well or bottom of a well. Such optically opaquematerials can reduce the background associated with optical detectionmethods. Optically opaque materials can be any known in the art or laterdeveloped, such as dyes, pigments or carbon black. The optically opaquematerial can prevent radiation from passing from one well to another, toprevent cross-talk between wells, so that the sensitivity and accuracyof the assay is increased. The optically opaque material can also bereflective, such as those known in the art, such as thin metal layers,mirror coatings, or mirror polish. Optically opaque materials can becoated onto any surface of the multiwell plate, or be an integral partof the plate or bottom as they are manufactured. Optically opaquematerial can prevent the transmittance of between about 100% to about50% of incident light, preferably between about 80% and greater than95%, more preferably greater than 99%.

[0187] Since most measurements will not typically require light to passthrough the wall of the well, materials such as polymers can includepigments to darken well walls or absorb light. Such application ofpigments will help reduce background fluorescence. Pigments can beintroduced by any means known in the art, such as coating or mixingduring the manufacture of the material or multiwell plate. Pigmentselection can be based on a mixture of pigments to dampen all backgroundinherent to the polymer, or a single pigment or ensemble of pigmentsselected to filter or absorb light at desired wavelengths. Pigments caninclude carbon black.

[0188] Surface electrodes can be embedded or otherwise attached to thewall in a variety of formats and arrangements, for example as severalnarrow vertical electrode stripes. By appropriately tuning the relativepotentials of each stripe, uniform electric fields can be generated inthe area of observation. Further, using a circular insert, or byembedding vertical stripe electrodes all around the well, uniformelectrical fields can be generated in any direction across the well. Itwould also be possible to create a uniform field in one direction,followed by a uniform field in another direction. This could be usefulfor cell types whose electrical characteristics are anisotropic, such asneural or muscle cells, or for cell types with large aspect ratios.

[0189] Each well also comprises a bottom having a high transmittanceportion and having less fluorescence than a polystyrene-bottom of atleast about 90 percent of said bottom's thickness. This property can bedetermined by comparing the fluorescence of an appropriate controlbottom material with the fluorescence of a test material. Theseprocedures can be performed using well known methods. Preferably, thebottom is a plate or film as these terms are known in the art. Thethickness of the bottom can vary depending on the overall propertiesrequired of the plate bottom that may be dictated by a particularapplication. Such properties include the amount of intrinsicfluorescence, rigidity, breaking strength, and manufacturingrequirements relating to the material used in the plate. Well bottomlayers typically have a thickness between about 10 micrometers and about1000 micrometers. Preferably, the well bottom has a thickness betweenabout 10 micrometers and 450 micrometers, more preferably between about15 micrometers and 300 micrometers, and most preferably between about 20micrometers and 100 micrometers.

[0190] The bottom of a well can have a high transmittance portion,typically meaning that either all or a portion of the bottom of a wellcan transmit light. The bottom can have an optically opaque portion anda high transmittance portion of any shape, such as circular, square,rectangular, kidney shaped, polygonal, or other geometric ornon-geometric shape or combinations thereof.

[0191] Preferably, the bottom of the multiwell plate can besubstantially flat, e.g. having a surface texture between about 0.001 mmand 2 mm, preferably between about 0.01 mm and 0.1 mm (see, SurfaceRoughness, Waviness, and Lay, Am. Soc. of Mech. Eng. #ANSI ASMEB46.1-2985 (1986)). If the bottom is not substantially flat, then theoptical quality of the bottom and wells can decrease because of alteredoptical and physical properties of one or both.

[0192] For surface electrode embodiments, the bottom will preferablycomprise strips of electrically conductive material or coatings thatoverlap the edge of the wells of the multiwell plate and are inelectrical contact with the contents of the wells. The electricallyconductive strips will typically terminate at electrical connectors toenable facile attachment to the output stage of a high-power functiongenerator as described previously. The electrically conductive stripsshould have low enough resistance so that they can carry the stimulatingcurrents without excessive loss in voltage over their length. Theresistance from the connector end to the farthest well end should beless than 10 Ω, and more preferably less than 1 Ω, and more preferablystill less than 0.1 Ω. The cross-sectional area of the electricallyconductive strips should be large enough to accomplish the resistancerequirement. For commonly employed electrical conductors, this crosssectional area should be at least 10⁻⁴ mm² , and more preferable atleast 10⁻³ mm².

[0193] In practice, any conductive materials could be used as long asthey are capped with a conductive material that is inert in saline. Suchmaterials include the noble metals (including gold, platinum, andpalladium) and the refractory metals (including chromium, molybdenum,iridium, tungsten, tantalum, and titanium) as well as alloys thereof.Preferred materials for the conductive material for surface electrodesinclude combinations of chromium, copper, gold, and indium-tin-oxidethat can be readily embedded or electroplated into or on the transparentbottom layer. Electrolysis products can be contained or eliminated bycoating the surfaces of the electrodes with protective coatings, such asgelatin, polyacrilimide, or agarose gels.

[0194] Another potentially useful electrode material is anelectrochemical half-cell, such as a silver/silver chloride electrode.

[0195] The electrically conductive material coatings or surfacemodifications can be introduced into the bottom using any suitablemethod known in the art, including vacuum deposition, electroplating,printing, spraying, radiant energy, ionization techniques or dipping.Surface modifications can also be introduced by appropriatelyderivatizing a polymer or other material, such as glass or quartz,before, during, or after the multiwell plate is manufactured and byincluding an appropriate derivatized polymer or other material in thebottom layer. The derivatized polymer or other material can then bereacted with a chemical moiety that is used in an application of theplate. Prior to reaction with a chemical moiety, such polymer or othermaterial can then provide either covalent or non-covalent attachmentsites on the polymer or other material. Such sites in or on the polymeror other material surface can be used to attach conductive layers to theplates. Examples of derivatized polymers or other materials includethose described by U.S. Pat. No. 5,583,211 (Coassin et al.) and othersknown in the art or later developed.

[0196] a. Materials and Manufacturing

[0197] The materials for manufacturing the multiwell plate willtypically be polymeric, since these materials lend themselves to massmanufacturing techniques. However, other materials can be used to makethe bottom of the multiwell plate, such as glass or quartz. The bottomcan be made of the same or different materials and the bottom cancomprise polystyrene, or another material. Preferably, polymers areselected that have low fluorescence and or high transmittance. Polymericmaterials can particularly facilitate plate manufacture by moldingmethods known in the art and developed in the future, such as insert orinjection molding.

[0198] The multiwell plate of the present invention can be made of oneor more pieces. For example, the plate and bottom can be made as onediscrete piece. Alternatively, the plate can be one discrete piece, andthe bottom can be a second discrete piece, which are combined to form amultiwell plate. In this instance, the plate and bottom can be attachedto each other by sealing means, such as adhesives, sonic welding, heatwelding, melting, insert injection molding or other means known in theart or later developed. The plate and bottom can be made of the same ordifferent material. For example, the plate can be made of a polymer, andthe bottom made of polystyrene, cycloolefin, Aclar, glass, or quartz.

[0199] Miniaturized surface electrode designs are feasible in standardplate formats (96, 384, 1536) as well as 3456 and higher platedensities. The throughput of such systems is potentially extremely high.For example, assuming 3456 wells per plate screened at 30 plates perhour corresponds to an overall throughput of approximately 800,000 wellsper eight-hour day, which is approximately 8 times faster than ispresently available, assuming equal plate read times.

[0200] An example of one embodiment of multiwell plate with surfaceelectrodes is shown in FIG. 2A. In this example, pairs of conductivestrips (200) are attached in parallel to an optically transparent bottomlayer (210) such as glass, or plastic such as COC (see U.S. Pat. No.5,910,287, issued Jun. 8, 1999) in a 96-well plate format. In thisexample, the strips of conductive material (200) are approximately 2 mmwide, 10 μm thick, and separated by distance of approximately 4 mm toenable optical analysis of the cells located in the wells (220), betweenthe electrodes through the optically transparent bottom layer (210). Inother embodiments the strips of conductive material can comprisestainless steel wires (from about 0.001″ to about 0.010″ diameter). Theoptically transparent bottom layer (210) is attached to a 96-wellmultiwell plate array (230) and replaces the normal plate bottom. Thestrips of electrically conductive material (200) overlap the edge of thewells (220) of the 96-well multiwell plate and are in electrical contactwith the contents of the wells. The electrically conductive strips (200)terminate at electrical contacts (240) to enable facile attachment tothe output stage of a high-power function generator as describedpreviously. In this example, there are two electrode contacts pereight-well column in the first well of the column. This permits the useof standard 96-well plate layouts, for simpler handling during cellculturing. No cells or saline are inserted into these wells. This designpermits the simultaneous stimulation of seven wells in a single column.During the assay, the operator or a robot will temporarily attach wiresto the contacts, for example with push-pin test electrodes.

[0201] Another embodiment of a multwell plate with surface electrodes isshown in FIG. 2b. In this embodiment, the transparent bottom layer (210)extends beyond the edge of the multiwell plate (230). In thisconfiguration, all wells remain available for use with cells andcompounds. Further, attachment of external wiring to the contacts (240)is simplified. Push-pin contacts, circuit-board edge connectors, orzero-insertion force sockets can be used to make contact with theelectrodes. The extended bottom layer (210) may make the plates lessconvenient to manipulate during routine use. This can be remedied bybringing the electrode traces (200) to the reverse side of the bottomlayer (210) during the manufacturing process. This can be accomplishedby several methods. For example, using two-sided processing of theplates to create contact areas, through-holes can be made andelectroplated, or conducting traces can be wrapped around the edge ofthe bottom layer. As another example, the bottom layer can be made of aflexible insulating material. Then, after making the structure as shownin FIG. 2B, the part of the bottom layer which protrudes from the edgeof the plate can be folded and attached to the underside of the plate.

[0202] Another embodiment of a multwell plate with surface electrodes isshown in FIG. 2C. In this embodiment, the electrodes (200) are attachedto the contact pads (240) with narrow via wires (205). This permits theuse of standard 96-well plate layouts, for simpler handling during cellculturing. In this embodiment, all of the electrodes of one polarity areshorted together. Selection of a single column is accomplished bysupplying the current pulse to only one electrode of the other polarity.In this embodiment, no cells, saline, or compounds are placed into thefinal column where the contact pads are. During the assay, the operatoror a robot will temporarily attach wires to the contacts, for examplewith push-pin test electrodes.

[0203] Another embodiment of a multwell plate with surface electrodes isshown in FIG. 2D. In this embodiment, the electrodes (200) are alignedparallel to the longer dimension of the 96-well plate. This design isessentially similar to the design shown in FIG. 2A, with the exceptionthat eleven wells in a row will be simultaneously stimulated.

[0204] Preferred materials for the conductive material for surfaceelectrodes include combinations of chromium, copper, gold, andindium-tin-oxide that can be readily embedded, attached, orelectroplated into or on the transparent bottom layer. In practice, anyconductive materials could be used as long as they are capped with aconductive material that is inert in saline. Such inert materialsinclude the noble metals (including gold, platinum, and palladium), therefractory metals (including chromium, molybdenum, iridium, tungsten,tantalum, and titanium), corrosion-resistant alloys (including stainlesssteel), and carbon or other organic conductors (including graphite andpolypyrrole) as well as combinations or alloys of these materials.

[0205] 4. Systems for Electrical Stimulation and SpectroscopicMeasurement

[0206] The present invention includes systems for automated electricalstimulation and spectroscopic measurement, comprising: at least oneelectrode assembly, a means for electrical stimulation, an opticaldetector, and computer control means to coordinate the generation ofelectrical stimuli, collection of data and movement of multiwell plates.The system can further comprise means for fluid addition. In one aspectthese systems are designed for modulating, characterizing and assayingthe activity of ion channels, transporters, leak currents present in oron the surfaces of living cells, and for rapidly screening for theeffects of test compounds on the effects of channel or cellularactivities. The present invention is also directed to chemical entitiesand information (e.g., modulators or chemical or biological activitiesof chemicals) generated or discovered by operation of workstations ofthe present invention.

[0207]FIG. 3 shows a block diagram of the major electrical and opticalcomponents for one embodiment of a system for automated electricalstimulation and spectroscopic measurement. In this example a 96-wellmultiwell plate dipper electrode array (FIG. 1) was used for electricalstimulation. In addition to the stimulator electrode array, the systemhas several additional electrical, optical and mechanical components, asdescribed in detail in commonly owned U.S. patent application Ser. No.09/118,728, filed Jul. 24, 1998.

[0208] In this embodiment, a National Instruments (Austin, Tex.) PC-DIO24 digital input/output card on board the computer (300) is used to setthe proper channel on a 1-to-12 switch (330) (National InstrumentsER-16). The computer controlling the fluorescent plate reader (300) alsosends out a TTL signal to trigger the function generators (310) when thestimulus is programmed to begin. Stimulus signals are generated by twoarbitrary waveform generators (310). The function generators areTektronix (Beaverton, Oreg.) model number AFG310. The first triggers aseries of TTL pulses to the second which is programmed with theindividual stimulus waveform. More complex waveform trains can begenerated by connecting multiple waveform generators in series and/or inparallel. These waveform generators would be triggered by thecomputer-generated TTL pulse or by each other. Alternatively, an A/Dconverter or a sound card on board the computer could be used togenerate a train of stimuli. In this case, commercially-available orcustom software could be used to program the waveform train, or tochange the waveform during the train.

[0209] The train of stimuli is sent through a high-power amplifier(320), through the switch (330), and into the stimulator head (370). Inthis case the amplifier was built using the APEX PA93 chip (ApexMicrotechnology Corp, Tucson, Ariz.) following a circuit provided by themanufacturer. Preferred amplifiers for the present application wouldtypically meet, or exceed the following specifications: ±100V DC in, 100GΩ input impedance, 20× voltage gain, ±90V out, ±3 A out, 10 Ω outputimpedance.

[0210] The majority of current passes through the saline between theelectrodes, typically in a single eight-well column of the microtiterplate (350) at a time. Excitation light at 400±7.5 nm illuminates thestained cells from below, and emitted fluorescent light is measured attwo wavelengths via the detector module (340) blue at 460+/−20 nm andorange at 580+/−30 nm; (see González et al., Drug Discovery Today 4:431-439, 1999). Once a column of cells has been stimulated the computer(300) triggers the motor (360) to move the multiwell plate (350) to anew position ready for the next stimulation.

[0211] For a typical 96-well multiwell plate, the electrodes are 4 mmwide with a gap (g) of 4 mm. Stimulation is usually performed in avolume of 100 μL of physiological saline in the well. With this volumeof saline, the depth averages approximately 3.0 mm (this depth varies byas much as 20% across the well due to the meniscus effect). Theelectrodes rest approximately 0.5 mm off the bottom of the wells. Theelectric field (E) applied across the cells is estimated as the voltageacross the electrodes (V₀) divided by the electrode gap (g), E=V₀/g .This is an overestimate of the actual field because of the influence ofelectrochemical reactions at each electrode which consume approximately1.5 V. In the typical voltage ranges used for stimulation (10 to 60V/cm), this overestimate is on the order of approximately 10%. Accuratemeasurement and calibration of the field can be performed by mapping theelectric potential in the well when current is passed.

[0212] The present invention also includes automated workstations thatare programmably controlled to minimize processing times at eachworkstation and that can be integrated to minimize the processing timeof the liquid samples for electrical stimulation and analysis.

[0213] Typically, a system of the present invention would include one ormore of the following: A) a storage and retrieval module comprisingstorage locations for storing a plurality of chemicals in solution inaddressable chemical wells, a chemical well retriever and havingprogrammable selection and retrieval of the addressable chemical wellsand having a storage capacity for at least 100,000 addressable wells, B)a sample distribution module comprising a liquid handler to aspirate ordispense solutions from selected addressable chemical wells, thechemical distribution module having programmable selection of, andaspiration from, the selected addressable chemical wells andprogrammable dispensation into selected addressable sample wells(including dispensation into arrays of addressable wells with differentdensities of addressable wells per centimeter squared), C) a sampletransporter to transport the selected addressable chemical wells to thesample distribution module and optionally having programmable control oftransport of the selected addressable chemical wells (including adaptiverouting and parallel processing), D) a system for automated washing,staining, and timed incubation of cells in multiwell plates, E) a systemfor automatically transporting cell plates and test compound platesbetween the various workstations, F) a system for automated electricalstimulation and spectroscopic measurement, and a data processing andintegration module, G) a master control system which co-ordinates theactivities of any of the above subsystems.

[0214] The storage and retrieval module, the sample distribution module,and the system for automated electrical stimulation and spectroscopicmeasurement are integrated and programmably controlled by the dataprocessing and integration module. The storage and retrieval module, thesample distribution module, the sample transporter, the system forautomated electrical stimulation and spectroscopic measurement and thedata processing and integration module are operably linked to facilitaterapid processing of the addressable sample wells. Typically, devices ofthe invention can process at least 100,000 addressable wells in 24hours. This type of system is described in U.S. Pat. No. 5,985,214,issued Nov. 16, 1999, which is incorporated herein by reference.

[0215] a. Microfluidic Systems

[0216] The present invention also includes the use of electrodes thathave been incorporated into microfluidic chips and which provide forhighly miniaturized electrical stimulation and analysis. Such systemsinclude those, for example, described in U.S. Pat. No. 5,800,690 issuedSep. 1, 1998 to Chow et al., European patent application EP 0 810 438 A2filed May 5, 1997, by Pelc et al. and PCT application WO 98/00231 filed24 Jun. 1997 by Parce et al. These systems typically use electrogenicfluid movement to manipulate small fluid volumes within microcapillariespresent on glass or silicon chips. These microfluidic chip basedanalysis systems can provide massively parallel miniaturized analysis.Such systems are preferred systems of spectroscopic measurements in someinstances that require miniaturized analysis.

[0217] For example, the microfabricated fluorescence-activated cellsorter described by Fu et al. (Nature Biotechnology 17: 1109-11, 1999)could be easily modified to have a pair of electrodes placed in, or nearthe optical interrogation region. Using the methods described herein,individual cells could be electrically stimulated and individuallysorted based on their response to the stimulation. This method wouldgreatly simplify the process of obtaining stable clones containing thedesired expression of channels. In another aspect, screening of testcompounds on single cells could be performed with a microfluidic deviceequipped with one or more additional fluid injection ports and one ormore embedded electrical stimulator devices built and operated based onthe methods described herein.

[0218] 5. Electrical Stimulation Methods

[0219] a. Introduction

[0220] Without being bound to any mechanism of action, the presentinventors provide the following description for the effect of electricalstimulation on cellular transmembrane potentials.

[0221] Typical voltage-dependent ion channels have a variety ofconducting and non-conducting states that are regulated by the localrelative transmembrane potential of the cell. By appropriately applyingexternal electrical fields to the cells, portions of the cell membranecan be driven to any desired transmembrane potential, thereby enablingthe regulation of the conduction states of voltage dependent ionchannels present within the cell. If the applied electrical field isappropriately varied, it is possible to sample a number of conductancestates of most ion channels, thereby cycling them through resting,activated, and inactivated states.

[0222] Depending on the ion channel in question, activation of the ionchannel can lead to the release, or uptake, of ions into the cell thatcan result in global transmembrane potential changes in the cell. Byapplying a repetitive train of electrical stimuli, separated by a timeinterval smaller than the membrane time constant, large sustainedmembrane voltage changes can be created via a stepwise accumulation orloss of ions. This process allows the direct measurement of many ionchannels and provides a facile method whereby the transmembranepotential of the cell can be externally controlled. This approachtherefore provides for improved methods of drug discovery that arecompatible with high throughput screening.

[0223] b. Overview of a Typical Stimulation Protocol

[0224] The simulated influence of a typical biphasic electricalstimulation protocol on a cell line expressing a voltage activatedsodium channel is illustrated, in simplified form, below. The followingdescription assumes that the cell line has no significant expression ofother ion channels, and that the resting transmembrane potential of thecell is above the threshold for inactivation of the sodium channel inquestion. In FIG. 4, the upper panel shows the time course of theapplied electrical field (E), the middle panel shows the simulatedinward sodium currents (I_(Na)) in response to the applied electricalfield, and the lower panel shows the idealized average transmembranepotential of the cell (V_(m)). In this example, the recordings relate tothe changes in these parameters that a single cell placed in the centerof the applied electrical field would be typically expected toexperience during an electrical stimulation wave train.

[0225] Referring to the first pulse, establishing the first electricalfield causes a potential drop across the cell that is maximal, withrespect to the resting transmembrane potential of the cell, at the edgesof the cell closest to the electrodes (see Hibino et al., BiophysicalJournal 64:1789-1800, 1993; Gross et al. 1986, Biophys. J. 50:339-348).The magnitude of the electric field-induced transmembrane potentialchange ΔV_(m) at a given point of the membrane in an idealized sphericalcell can be described by the formula (Ehrenberg et al., Biophys. J.51:833-837, 1987):

ΔV _(in) =−fgrE cos θ.  (1)

[0226] In Equation 1, ƒ is a factor dependent upon the conductivity ofthe membrane, g is a geometric factor of order 1, r is half the diameterof the cell parallel to the electric field, E is the local magnitude ofthe electric field, and θ is the angle between the local direction ofthe field and a line drawn from the center of the cell to the point ofthe surface being considered. For most intact mammalian cells, in whichthe membrane conductivity is very low compared to the conductivity ofthe solution bathing the cells, the factor ƒ≈1. In practice, cells arerarely spherical when attached to a substrate and an accurate estimateof the actual magnitude of the electrical field induced transmembranepotential changes may be empirically determined.

[0227] As a result of the applied electrical field, the membrane on theside nearest to the anode is driven negative, while the membrane on theside nearest the cathode is driven positive. In cells in which one edgeis driven sufficiently negative to locally lower the transmembranepotential below the threshold potential for release of inactivation forthe ion channel in question, the applied electrical field causes thesodium channels located on this edge to enter the resting state. On theother side of the cell, the transmembrane potential is driven positiveof the resting potential. Because the resting transmembrane potential ofthe cell is assumed to be above the threshold for inactivation, sodiumchannels on this side of the cell remain inactivated and do not passcurrent. If the resting transmembrane potential were instead below theinactivation threshold, channels on this side of the cell would activateand pass current.

[0228] When the applied field is reversed, the profile of transmembranepotential changes also reverses. The transmembrane potential changesinduced by the electric field on the patches of membrane at the extremeedges of the cells switches polarity. The channels on the side that wasdriven negative during the first phase of stimulation are now drivenpositive. If the stimulation parameters are properly chosen, thesechannels are now driven above the activation potential and begin toallow sodium ion influx. This is shown in FIG. 4, as the first smallerpeak of sodium influx into the cell. The sodium channels rapidlyinactivate after a characteristic time. Meanwhile, on the other side ofthe cell, the transmembrane potential is driven negative so that thesodium channels release from inactivation and move into the restingstate.

[0229] When the second stimulus phase ends, all parts of the membranerapidly return to a new average transmembrane potential. If the averagetransmembrane potential is now above the activation potential of thesodium channels, the channels on the side of the cell that was drivennegative during the second phase of stimulation activate and begin toallow sodium ion influx. This is shown in FIG. 4, as the second largerpeak of sodium influx into the cell. The sodium channels rapidlyinactivate after a characteristic time. In this case sodium influx istypically larger from the second side than the first side, since thedriving force for sodium entry is larger when this part of the membraneis driven more positive by an electric field.

[0230] Each pulse of sodium channel influx raises the averagetransmembrane potential of the cell (FIG. 4, lower panel). This rise intransmembrane potential can be detected by any of the methods describedherein, but is conveniently measured via fluorescence emission ratiochanges of a FRET based voltage-sensitive dye. Due to leakage currentspresent in all cells, this average transmembrane potential shift decaysexponentially to the original resting transmembrane potential. The timedependency of this response, the membrane time constant (τ_(m)), dependsupon the membrane capacitance and membrane resistance, and is highlyvariable from one cell type to another. For example, time constants canvary from 100 μs to over one second, depending on the cell type.Typically the membrane time constant is around 100 ms for mostengineered cell lines. To provide a net accumulation of sodium influxthe stimulus pulse is repeated before the transmembrane potential hastime to decay to the resting transmembrane potential.

[0231] During subsequent rounds of electrical stimulation, positivecharge is steadily accumulated into the cell raising the averagetransmembrane potential in an approximately stepwise fashion with eachrepetition of electrical stimulation. After each pulse of electricalstimulation, the magnitude of the sodium ion influxes become steadilysmaller as the average transmembrane potential approaches the sodium ionreversal potential. Eventually an equilibrium transmembrane potential isestablished in which leakage of current out of the cell equals thecurrent influx due to electrical stimulation.

[0232] c. Adjustable Parameters for the Stimulus Waveforms

[0233] The present invention includes the use of any waveform kernelwith any repetition procedure capable of selectively activating ionchannels in living cells. The kernel is the repeatable structure thatforms the basis of the stimulus train. In FIG. 4, the kernel is abiphasic square pulse, but in principle it could be any limited-timewave function. The time duration of the kernel sets the maximum rate atwhich it can be repeated. The repetition procedure dictates how and whenthe kernel is presented to the sample. In FIG. 4, the repetition rate isfixed and continues for a total of ten cycles. However the repetitionrate need not be fixed.

[0234] Additionally, the kernel can be changed during the stimulustrain, so that each time the repetition procedure calls for a stimuluspulse, a different wave function could be used. Furthermore, a feedbackmechanism could be used to alter the kernel and/or the repetitionprocedure based upon the measured response of the system.

[0235] The use of arbitrary waveform generators to create the stimuluskernels and trains allows for a virtually unlimited variation in thewaveform in order to tune the electrical stimulus to a particular celltype or specific ion channel. The pulse train can be readily modulatedvia the variation of a number of separately controllable components.

[0236] i. The Shape of the Individual Pulses.

[0237] The waveform kernel that is repeated during the stimulus traincan be changed with nearly endless permutations using a arbitrarydigital waveform generator, such as Tektronix AFG 310. FIG. 5 shows aschematic representation of a biphasic square waveform to illustratesome of the variables that can be modulated. In FIG. 5, the pulse trainconsists of a starting field E₁ (400), that lasts for a time t₁, a rapidincrease in potential (410), that takes a time t₂, until reaching afirst stimulating field E₂ (420) that lasts for time t₃, a rapiddecrease in potential (430) that takes time t₄, until reaching a secondstimulating field (440), E₃ that lasts a time t₅, a rapid increase inpotential (460) that takes time t₆, until reaching the finishing field(470), E₄ that lasts a time t₇ until the cycle is repeated. Themagnitude and polarity of the electrical fields E₁ to E₄ are separatelycontrollable and may be both statically and dynamically varied asdescribed below. The times for which the electrical potentials areapplied to the cells, times t₁, t₃, t₅, and t₇ are also separatelycontrollable and may be both statically and dynamically varied between 0and 10 s during a wave train, as described below. Finally the changes inpotential that occur over times t₂, t₄ and t₆, may occur over variabletime periods between 0 and 100 ms and be either linear or non linear tocreate waveforms of variable shapes.

[0238] Some examples of these types of variation in the waveform areshown in FIG. 6 (a) Biphasic waveform, as shown in FIG. 5, repeated at arate f. (b) A modified biphasic waveform. A short interval has beenadded between the stimulation phases of the wave train. This allowscurrent to flow through the channels released from inactivation duringthe first pulse. (c) Monophasic waveform. Only channels on the side ofthe cell facing the anode will be released from inactivation. (d) Aramped waveform. The anode-facing channels will be released frominactivation by the square wave. The channels will activate and passcurrent during the ramp. The ramp allows the channels to open and passcurrent at more negative local potentials, so that even when the cell isnear the reversal potential for sodium ions, large currents can stillflow. The point along the ramp at which the channels will open varies.(e) A biphasic triangular or sawtooth waveform. Ramping may allow thevoltage-dependent transitions between states to occur more uniformly asthe global membrane potential changes. Monophasic triangular waveformsare also possible. (f) A sinusoidal waveform. This type of waveform mayreduce electrical noise during high frequency stimulation. (g) A shortburst of sinusoidal waveforms. (h) Bursts of sinusoidal waveforms, eachwith different fundamental frequency. This type of stimulation may proveuseful for studying plasticity effects. The first burst(s) are used totrain the system or begin a process, while the subsequent bursts(s) areused to assay the system.

[0239] Variations in waveform shape may be useful in maintaining fixedstimulus conditions during the pulse train. For example, thetransmembrane potential excursions experienced by a highly polarizedcell will vary as its average transmembrane potential changes fromaround −90 mV at the beginning of the stimulation cycle to around +60 mVafter several repetitive stimulation cycles. As a consequence, theapplied electrical field required to efficiently release an ion channelfrom inactivation varies as the average potential of the cell variesduring the course of several stimulation cycles. To take this effectinto consideration it may be useful, under certain circumstances, tochange the relative balance between the positive (E₂) and negative (E₃)phases of stimulation as the wave-train progresses.

[0240] Some cell lines, for example HEK-293, have a resting averagetransmembrane potential below the activation threshold of somevoltage-activated sodium channels. In these cells as the transmembranepotential rises during stimulation as a result of sodium ion influx, thesodium channels can open independently of the applied electricalstimulation. This can be improved by using a sloped current pulse (i.eby increasing t₂ and t₄). Then, the channels can pass current for adefined time just above the activation voltage, independent of theaverage transmembrane potential of the cell.

[0241] ii. The Overall Amplitude of the Individual Pulse (E₂ and E₃).

[0242] The magnitude and polarity of the pulse amplitude controls therelative transmembrane potential excursions experienced by the cellduring a stimulus pulse. Pulse amplitudes can be altered for the entiretrain, or for the individual pulses to accommodate different channelsand cell types, as discussed in more detail below. In general, themagnitudes of E₂ and E₃ are selected to ensure that the ion channel ofinterest is efficiently activated, and released from inactivation duringeach stimulation cycle, while at the same time not of sufficientmagnitude so as to cause irreversible electroporation of the cells.Preferred pulse amplitudes for E₂ and E₃ are typically in the range of 5to 60 V/cm for most ion channels when expressed in non-excitablemammalian cells with average sizes from 10 to 25 μm, and may vary eitherpositive or negative relative to earth. As above, the amplitude of thestimulus can be changed during the pulse train to maintain stablestimulus conditions as the average transmembrane potential changes.

[0243] Preferred pulse amplitudes are inversely dependent upon averagecell size. So, the technique can also be used on cells which are smalleror larger than 10 to 25 μm, by altering the pulse amplitude.

[0244] iii. The Duration of the Individual Pulses (t₃ and t₅).

[0245] Many channels require alterations in the transmembrane potentialfor extended periods of time to release them from inactivation, prior toopening. For example, many voltage-dependent sodium channels generallyneed to experience a transmembrane potential below −90 mV for severalmilliseconds before they are released from inactivation. Efficient useof the electrical stimulation protocol therefore typically requires thatthe duration of the pulses t₃ and t₅ are sufficient to enable complete,or almost complete, release from inactivation for the ion channel ofinterest. In some cases it may be desirable to tune the magnitude of t₃and t₅ to enable the selective release from inactivation of one class,but not another class of ion channel in a cell that expresses severalion channel types. In other cases it may be desirable to make t₃ and t₅very small to achieve low levels of release from inactivation for thechannels. Typically the preferred pulse duration is matched to thecharacteristic time for transitions between the desiredvoltage-dependent states for the ion channel of interest, and these aretypically in the range of about 0.1 to 100 msec for most ion channels.

[0246] To avoid excessive electrolysis of water and consequent gasbubble generation, the duration of the pulses t₃ and t₅ should be keptas short as possible, while still achieving the desired electricalstimulation. Water electrolysis at a metal/water interface typicallyoccurs when the magnitude of the voltage difference between the metaland the water exceeds about 0.8 V. In some cases, the stimulusparameters required to produce cellular stimulation also cause waterelectrolysis. Some generation of gas at the electrodes is typicallyacceptable as long as the charge per unit area of the electrode/waterinterface delivered during any single polarity phase of a single pulseis less than about 100 μC/mm². Exceeding this limit typically causes gasevolution and bubble formation that significantly affects fielduniformity. The presence of bubbles on the electrode surface occludesthat part of the electrode, and can cause alterations in the electricfield uniformity. Generation of large amounts of gas can also causeoxidative damage to the cells and the dyes in the well.

[0247] In a 96-well plate with 100 μL of physiological saline withresistivity 70 Ω-cm, the resistance of the saline between two parallelplate electrodes with a 4 mm gap between them inserted into the well towithin 0.5 mm of the bottom of the well, is approximately 230 Ω. Eachelectrode has a contact area with the saline of about 24 mm². Thus, anysingle-polarity phase of the stimulus protocol should not deliver morethan about 2.4 mC of charge. A voltage difference of about 10 V appliedbetween the plates generates an electric field of about 25 V/cm in thesaline. This voltage will draw about 43 mA of current. Thus for thiselectrode configuration, a square wave, single-polarity pulse should notexceed about 55 milliseconds in duration in order to limit the charge toless than 2.4 mC.

[0248] iv. The Gap Between Successive Stimuli (t₁ and t₇).

[0249] Changing the value of t₁ and t₇ globally for the train, oradjusting it for each individual pulse during the train, is useful foroptimizing the stimulation protocol for specific ion channels.Additionally the approach is also useful for determining certaincellular and channel properties including the open channel time and thetime course of the channel activation and inactivation.

[0250] For example, for assays involving voltage regulated sodiumchannels, the insertion of a time delay (t₁+t₇) between pulses equal to,or less than, the average sodium channel open time allows for aquantitative measurement of the inactivation kinetics of the channel.The inactivation kinetics are directly related to the average openchannel time. Thus, assays using short interpulse intervals allows forthe detection of compounds whose primary effect is on inactivationkinetics, a mechanism which is otherwise inaccessible usinghigh-throughput techniques.

[0251] In most cases the time delay between successive stimuli would beless that the membrane time constant in order to obtained sustainedincreases in transmembrane potential. Typically optimal frequencies ofstimulation (ƒ) are within the range τ_(M) ⁻¹≦ƒ≦τ_(b) ⁻¹ where τ_(M) isthe time constant for decay of transmembrane potential changes, andτ_(b) is the average channel open time. Some channels do not inactivate,and for these cells the stimulation frequency may be determinedempirically. Additionally, the stimulation frequency ƒ cannot exceed theinverse of the time duration of the stimulus kernel.

[0252] Additionally, for certain cell types, it may prove desirable tostimulate at a slower rate. For example, slower stimulation rates may bepreferred for cells with high channel densities, or for assays in whichhigher pharmacological sensitivity is required. Alternatively for thesecases, a monopolar stimulus could be used. This would only release frominactivation the sodium channels on one side of the cell, but themaximum frequency of stimulation could be doubled.

[0253] v. The Duration of the Train of Pulses, or Number of Pulses inthe Train.

[0254] Cellular and channel properties can be assayed both in dynamic(i.e. rise and fall times, alterations in response shape, etc.) andstatic modes. Both modes require stimulus train durations long enough toexplore all the events of interest, yet not longer than necessary tocomplete the assay. Typical stimulation times comprise 10 msec pulses,at 25 V/cm pulses repeated at a frequency of 20 Hz for 3 seconds.Adjusting these parameters allows assay times to be reduced, or toexplore processes with both fast and slow time scales.

[0255] vi. Multiple Pulse Trains.

[0256] In some cases it is useful to repeat pulse trains, or to performa measurement on the same cells with two different pulse trains. Oneexample would be to completely characterize the properties of a channelby measuring the response as a function of stimulus frequency andduration, using a single plate of cells subjected to multiple stimulustrains. Another example would be to examine plasticity of the response(i.e. activity-dependent changes in response). One or more stimulustrains would condition the response, while sets of measurement trainsbefore and after the conditioning would determine the changes due toactivity.

[0257] vii. Feedback of Stimulus Parameters Based Upon DynamicMeasurements of the Response.

[0258] The present invention can also be used to create a voltage clampdevice, by using a dynamic feedback loop to maintain the averagetransmembrane potential at a preset value. By measuring thetransmembrane potential using a fast fluorescent output as describedbelow, then changing stimulus parameters to compensate for any changesin transmembrane potential, it is possible to dynamically control thetransmembrane potential of the cells. The current necessary to maintainthat potential would then be determined by computer control of thestimulus parameters.

[0259] viii. The Use of High Frequency Stimulation to Avoid Electrolysis

[0260] During typical stimulation parameters, a peak current ofapproximately 50 mA passes through the solution between the electrodes.During this time various electrochemical reactions occur which typicallygenerate toxic species to the cells. Preliminary experiments have shownthat most mammalian cells typically respond normally for approximatelytwo minutes of electrical stimulation using stainless steel electrodes.However prolonged stimulation for longer time periods appears to lead toa loss in cell health and viability. At sufficiently high pulsefrequencies, such that the metal-saline interface does not reach thepotential for electrolysis of water (approximately ±1V for stainlesssteel in saline), current can be passed capacitively and no toxicproducts will be generated. In the electrical stimulator shown in FIG.1, in which each electrode has an area of about 24 mm² in contact withthe saline, the capacitance per electrode is around 1-10 μF (Robinson,1968, Proc. IEEE 56:1065-1071). At 50 mA, this capacitance charges to 1V in around 20-200 μs. This is at the lower limit of the useful pulseduration times.

[0261] Alternatively it is possible to perform electrical stimulationwithout generating electrolytic products. Several treatments areavailable which can increase the capacitance of the metal-salineinterface by factors of 2-100. These include surface roughening,electroplating with platinum black or gold black, and deposition andactivation of iridium/iridium oxide, titanium/titanium nitride, orpolypyrrole films. Using stimulation parameters, which avoidirreversible electrochemistry, these surface treatments do not degradewhen passing current.

[0262] 6. Expression of Ion Channels

[0263] a. Selection of the Cell Type

[0264] The present invention can be used with any type of cell,including animal cells, plant cells, insect cells, bacterial cells,yeast and mammalian cells. For screening for human therapeuticsmammalian cell lines are preferred, such cell lines include tissueculture cell lines that can be relatively easily grown, and can bereadily transfected with high efficiency. Many tissue cell lines arecommercially available through the American type culture collection(ATCC) see (http://www.atcc.org), as well as the European collection ofcell cultures (ECACC) (http://www.camr.org.uk).

[0265] Additionally in some cases primary cell lines, or tissue slicesmay also be preferred for screening when it is required to express, ormeasure, the response of the ion channel of interest in its nativephysiological context. This approach may be useful either as a primaryor a secondary screen to screen for specificity, selectivity or toxicityof candidate therapeutics, and is discussed in detail in section X.

[0266] For assays performed on cultured cell lines, the main selectioncriteria are the resting transmembrane potential of the cell line, andthe presence of endogenously expressed ion channels. The selection ofappropriate cell lines for specific ion channels of interest aredependent on the voltage dependent properties and ion selectivity of theion channel of interest. These considerations are reviewed in detail fora number of ion channels in section VIII, Stimulation Protocols.

[0267] In some cases it is desirable to use a cell line which has no (orvery low) detectable endogenous expression of other ion channels. Cellsof this type include CHO-K1, CHL, and LTK(−) cells. These cellsinherently have a resting potential in the range of +10 to −30 mV, whichis above the activation and inactivation thresholds of mostvoltage-dependent channels. Use of these cell lines has the advantagethat the ion channel of interest is the major modulator of transmembranepotential within the cells so that screening assay data are generallyeasily and unambiguously interpreted.

[0268] In some cases the use of a cell line with no other ion channelsmay not be practical to create a workable assay. For example, it may benecessary to maintain a voltage-regulated ion at a highly polarizedtransmembrane potential. In this case it is necessary control thetransmembrane potential via the expression of a second ion channel. Forexample to assay a rat brain type IIa sodium channel in the restingstate requires the transmembrane potential to be maintained below thethreshold activation potential of the sodium channel, in this casearound −60 mV. To achieve this it is necessary to either co-express anion channel, such as a potassium inward rectifier, that can maintain theresting transmembrane potential of the cell to around −90 mV, oridentify a cell line that endogenously expresses similar ion channels.Cell types of this type include RBL cells and HEK-293 cells.

[0269] In other cases it may be necessary to use the expression of asecond ion channel, in conjunction with electrical stimulation to drivethe cell membrane to a specific transmembrane potential, to enable thefirst ion channel of interest to be assayed. Examples of this situationoccur when assaying non-voltage regulated ion channels such asligand-gated channels. Co-expression of a voltage regulated sodiumchannel, for example in conjunction with electrical stimulation can beused to set the transmembrane potential to transmembrane potentials ofbetween about +10 to +60 mV. By comparison, co-expression of voltageregulated potassium channels in conjunction with electrical stimulationcan set the transmembrane potential to transmembrane potentials ofbetween about −90 to −30 mV. These approaches thus enable the effectivemanipulation of the transmembrane potential over a relatively wide rangethereby enabling the analysis of virtually any ion channel.

[0270] Typically when using this co-expression approach it is necessaryto re-screen any hits obtained with the cell line co-expressing both ionchannels, with the cell line expressing only the ion channel used to setthe transmembrane potential. This enables drugs that affect this secondion channel to be differentiated from those that actually influence theion channel of interest. Alternatively selective toxins such as TTX canbe used to selectively inhibit one class of ion channel.

[0271] b. Transfection of Ion Channels

[0272] Nucleic acids used to transfect cells with sequences coding forexpression of the ion channel of interest are typically in the form ofan expression vector including expression control sequences operativelylinked to a nucleotide sequence coding for expression of the channel. Asused, the term “nucleotide sequence coding for expression of a channel”refers to a sequence that, upon transcription and translation of mRNA,produces the channel. This can include sequences containing, e.g.,introns. As used herein, the term “expression control sequences” refersto nucleic acid sequences that regulate the expression of a nucleic acidsequence to which it is operatively linked. Expression control sequencesare operatively linked to a nucleic acid sequence when the expressioncontrol sequences control and regulate the transcription and, asappropriate, translation of the nucleic acid sequence. Thus, expressioncontrol sequences can include appropriate promoters, enhancers,transcription terminators, a start codon (i.e., ATG) in front of aprotein-encoding gene, splicing signals for introns, maintenance of thecorrect reading frame of that gene to permit proper translation of themRNA, and stop codons.

[0273] Methods which are well known to those skilled in the art can beused to construct expression vectors containing the ion channel codingsequence, operatively coupled to appropriate localization or targetingdomains and appropriate transcriptional/translational control signals.For example by reference to the sequence accession numbers, orreferences in Tables 1 to 3, one or ordinary skill in the art canidentify the sequence of the ion channel of interest. These methodsinclude in vitro recombinant DNA techniques, synthetic techniques and invivo recombination/genetic recombination. (See, for example, thetechniques described in Maniatis, et al., Molecular Cloning A LaboratoryManual, Cold Spring Harbor Laboratory, N.Y, 1989). Many commerciallyavailable expression vectors are available from a variety of sourcesincluding Clontech (Palo Alto, Calif.), Stratagene (San Diego, Calif.)and Invitrogen (San Diego, Calif.) as well as and many other commercialsources.

[0274] A contemplated version of the method is to use induciblecontrolling nucleotide sequences to produce a sudden increase in theexpression of the ion channel of interest e.g., by inducing expressionof the channel. Example inducible systems include the tetracyclineinducible system first described by Bujard and colleagues (Gossen andBujard (1992) Proc. Natl. Acad. Sci USA 89 5547-5551, Gossen et al.(1995) Science 268 1766-1769) and described in U.S. Pat. No. 5,464,758.

[0275] Transformation of a host cell with recombinant DNA may be carriedout by conventional techniques as are well known to those skilled in theart. Where the host is prokaryotic, such as E. coli, competent cellsthat are capable of DNA uptake can be prepared from cells harvestedafter exponential growth phase and subsequently treated by the CaCl₂method by procedures well known in the art. Alternatively, MgCl₂ or RbClcan be used. Transformation can also be performed after forming aprotoplast of the host cell or by electroporation.

[0276] When the host is a eukaryote, such methods of transfection of DNAas calcium phosphate co-precipitates, conventional mechanical proceduressuch as microinjection, electroporation, insertion of a plasmid encasedin liposomes, or virus vectors may be used. Eukaryotic cells can also beco-transfected with DNA sequences encoding the ion channel, and a secondforeign DNA molecule encoding a selectable phenotype, such as the herpessimplex thymidine kinase gene. Another method is to use a eukaryoticviral vector, such as simian virus 40 (SV40) or bovine papilloma virus,to transiently infect or transform eukaryotic cells and express the ionchannel. (Eukaryotic Viral Vectors, Cold Spring Harbor Laboratory,Gluzman ed., 1982). Preferably, a eukaryotic host is utilized as thehost cell as described herein.

[0277] Selection of stable clones will typically be made on the basis ofsuccessful expression of the ion channel of interest at sufficient levelto enable it's facile detection. In many cases this analysis willrequire functional characterization of individual clones to identifythose that exhibit appropriate electrophysiological characteristicsconsistent with expression of the clone of interest. This analysis canbe completed via the use of patch clamping, or via the measurement oftransmembrane potentials using transmembrane potential sensitive dyes asdescribed below. An advantage to the use of this latter method is thatit is compatible with fluorescence activated cell sorting and providesfor the rapid analysis of many thousands of individual clones persecond. In some cases where the sodium channel is electrically silent inthe resting cell, confirmation of expression can also be readilyachieved by immunochemistry using antibodies raised against the nativeion channel, or a defined epitope introduced in the ion channel viamolecular techniques as described above.

[0278] In cases where cells are transfected with a first ion channel ofinterest, and a second ion channel to set the transmembrane potential,optimization of the relative expression of both ion channels isimportant. Typically the optimal relative expression of the two ionchannels is determined empirically by selecting clones that provide themaximum dynamic range and minimal statistical variation in theirresponse.

[0279] 7. Measurement of Transmembrane Potentials

[0280] Transmembrane potential changes and the measurement of specificion channels conductance via the use of the present invention can bedetected by use of any of the known means of measuring transmembranepotential or ion movement. These methods include, for example, patchclamping (Hamill et al, Pfluegers Arch. 391:85-100, 1981), FRET basedvoltage sensors, electrochromic transmembrane potential dyes (Cohen etal., Annual Reviews of Neuroscience 1: 171-82, 1978), transmembranepotential redistribution dyes (Freedman and Laris, Spectroscopicmembrane probes Ch 16, 1988), extracellular electrodes (Thomas et al.,Exp. Cell Res. 74: 61-66, 1972), field effect transistors (Fromherz etal., Science 252: 1290-1293, 1991) , radioactive flux assays, ionsensitive fluorescent or luminescent dyes, ion sensitive fluorescent orluminescent proteins, the expression of endogenous proteins or the useof reporter genes or molecules.

[0281] Preferred methods of analysis for high throughput screeningtypically involve the use of optical readouts of transmembranepotential, or ion channel conductance. Such methods include the use oftransmembrane potential or ion sensitive dyes, or molecules, thattypically exhibit a change in their fluorescent or luminescentcharacteristics as a result of changes in ion channel conductance ortransmembrane potential.

[0282] A preferred optical method of analysis for use with the presentinvention has been described in U.S. Pat. No 5,661,035 issued Aug. 26,1997, hereby incorporated by reference). This approach typicallycomprises two reagents that undergo energy transfer to provide aratiometric fluorescent readout that is dependent upon the transmembranepotential. Typically the approach uses a voltage sensing lipophilic dyeand a voltage insensitive fluorophore associated with a cell membrane.(see Gonzalez et al. Drug Discovery Today 4:431-439, 1999).

[0283] In one embodiment, two dye molecules, a coumarin-linkedphospholipid (CC2-DMPE) and an oxonol dye such asbis-(1,2-dibutylbarbituric acid) trimethine oxonol [DiSBAC₄(3)], areloaded into the plasma membrane of cells. CC2-DMPE partitions into theouter leaflet of the plasma membrane where it acts as a fixed FRET donorto the mobile, voltage sensitive oxonol acceptor. Cells with relativelynegative potentials inside will push the negatively charged oxonol tothe outer leaflet of the plasma membrane, resulting in efficient FRET(i.e. quenching of the coumarin donor and excitation of the oxonolacceptor). Depolarization results in rapid translocation of the oxonolto the inner surface of the plasma membrane, decreasing FRET. BecauseFRET can only occur over distances of less than 100 Å, excitation of thecoumarin results in specific monitoring of oxonol movements within theplasma membrane.

[0284] The response times for these assays is readily altered byincreasing or decreasing the hydrophobicity of the oxonol. For example,the more hydrophobic dibutyl oxonol DiSBAC₄(3) has a time constant ofapproximately 10 ms, significantly faster than the less hydrophobicdiethyl oxonol DiSBAC₂(3).

[0285] Loading of the dyes is typically achieved at room temperatureprior to the start of transmembrane potential measurements. Typicallycells are loaded sequentially with the coumarin lipid followed by theoxonol. Typical loading concentrations for coumarin lipids range fromabout 4 to 15 μM (final concentration) and staining solutions aretypically prepared in Hanks Balanced salt solution with 10 mM HEPES, 2g/L glucose and about 0.02% Pluronic-127 at a pH of around 7.2 to 7.4.Loading is usually acceptable after about 30 minutes incubation, afterwhich excess dye may be removed if desired. Oxonol dyes are typicallyloaded at a concentration between 2 and 10 μM for 25 minutes at roomtemperature, the more hydrophobic DiSBAC₄(3) is usually loaded in thepresence of 2-3 μM Pluronic-127. Optimal loading concentrations varybetween cell types and can be empirically determined by routineexperimentation. Typically such optimization experiments are conductedby systematically titrating the concentrations of the first reagent, andthen for each concentration tested, titrating the concentration of thesecond reagent. In this way it is possible to obtain both the optimalloading concentrations for each reagent, and the optimal relative ratioto achieve a maximal signal to noise ratio.

[0286] In some cases it may be preferred to add, or load one, or more ofthe FRET reagents with one or more light absorbing substances in orderto reduce undesired light emission, as for example described in commonlyowned U.S. patent application Ser. No. 09/118,497, filed Jul. 17, 1998;U.S. patent application Ser. No. 09/120,516, filed Jul. 21, 1998, andU.S. patent application Ser. No. 09/122,477 filed Jul. 23, 1998.

[0287] FRET based voltage sensors may also be derived from the use ofother membrane targeted fluorophores in conjunction with a mobilehydrophobic donor or acceptor. Other such compositions are disclosed,for example, in U.S. patent application Ser. No. 09/459,956, filed Dec.13, 1999.

[0288] Suitable instrumentation for measuring transmembrane potentialchanges via optical methods includes microscopes, multiwell platereaders and other instrumentation that is capable of rapid, sensitiveratiometric fluorescence detection. A preferred instrument of this typeis described in U.S. patent application 09/118,728 filed Jul. 17, 1998.This instrument (the Voltage/Ion Probe Reader or VIPR™) is an integratedliquid handler and kinetic fluorescence reader for 96-well and greatermultiwell plates. The VIPR™ reader integrates an eight channel liquidhandler, a multiwell positioning stage and a fiber-optic illuminationand detection system. The system is designed to measure fluorescencefrom a column of eight wells simultaneously before, during and after theintroduction of liquid sample obtained from another microtiter plate ortrough. The VIPR™ reader excites and detects emission signals from thebottom of a multiwell plate by employing eight trifurcated opticalbundles (one bundle for each well). One leg of the trifurcated fiber isused as an excitation source, the other two legs of the trifurcatedfiber being used to detect fluorescence emission. A ball lens on the endof the fiber increases the efficiency of light excitation andcollection. The bifurcated emission fibers allow the reader to detecttwo emission signals simultaneously and are compatible with rapidsignals generated by the FRET-based voltage dyes. Photomultiplier tubesthen detect emission fluorescence, enabling sub-second emission ratiodetection.

[0289] 8. Stimulation Protocols

[0290] In one aspect, the present invention includes methods formodulating the transmembrane potentials of living cells via electricalstimulation, and the use of these methods for assaying the activity ofvirtually any ion channel or transporter system.

[0291] a. Measurement of Specific Channel Conductances

[0292] i. Assay of Sodium Channels

[0293] A variety of different isoforms of mammalian voltage dependentsodium channels have been identified, and are summarized below inTable 1. These channels can be classified into three main groups (forreview see Goldin, Annals N.Y. Academy of Sciences 868:38-50, 1999).TABLE 1 Sodium Channel Sub-type Summary Channel Name Sub-type/ & GeneAlternate Tissue Accession Symbol names Distribution Number SCN1A(Nav1.1) Rat I (rat) CNS/PNS X03638 HBSCI (human) CNS X65362 GPB1(Guinea pig) CNS AF003372 SCN2A (Nav1.2) Rat II (rat) CNS X03639 HBSCII(human) CNS X65361 HBA (human) CNS M94055 Nav 1.2A Rat IIA CNS X61149SCN3A (Nav 1.3) Rat III (rat) CNS Y00766 SCN4A (Nav1.4) SkM1, μ1 (rat)skeletal muscle M26643 SkM1 (human) Skeletal muscle M81758 SCN5A(Nav1.5) SkM2 (rat) skeletal muscle/ M27902 RH1 (rat) heart H1 (human)heart M77235 SCN8A (Nav1.6) NaCh6 (rat) CNS/PNS L39018 PN4a (rat)CNS/PNS AF049239A Scn8a (mouse) CNS U26707 Scn8a (human) CNS AF050736CerIII (Guinea pig) CNS AF003373 SCN9A (Nav1.7) PN1 (rat) PNS U79568HNE-Na (human) thyroid X82835 Nas (rabbit) Schwann cells U35238 SCN10ANav1.8 SNS (rat) PNS X92184 PN3 (rat) PNS U53833 SNS (mouse) PNS Y09108SCN6A Nav2.1 Na2.1 (human) Heart, uterus M91556 muscle SCN7A Nav2.2 Na-G(rat) astrocytes M96578 SCL11 (rat) PNS Y09164 Nav2.3 Na2.3 (mouse)Heart, uterus L36179 muscle Nav3.1 NaN (rat) PNS AF059030 SCN1B Naβ1.1β-1 (rat) CNS M91808 β-1 (human) CNS L10338 SCN2B Naβ2.1 β-2 (rat) CNSU37026 β-2 (human) CNS AF007783

[0294] The voltage-dependent sodium channels in Table 1 vary widely intheir voltage dependency and inactivation and activation kinetics.Voltage-gated sodium channels have many different conformations, whichcan be classified into three states. (1) The resting state, in which thechannel is closed and no current can flow. This is the typical statewhen a sodium channel is expressed in a cell with a restingtransmembrane potential of below about −60 mV. The channel can berapidly driven into the open state by depolarization, usually to atransmembrane potential of above about −50 mV. (2) The activated state,in which the channel is open and ions can pass through. Because theintracellular concentration of sodium is low in a normal resting cell,while the extracellular concentration is high, sodium ions flow into thecell and drive the transmembrane potential more positive. The open statehas a short lifetime, generally on the order of one millisecond, afterwhich it passes into the inactivated state. (3) The inactivated state,in which a channel has closed and ions can not pass through the channel.The channel cannot be directly opened once in the inactivated state. Itwill first go to the resting state, which occurs if the transmembranepotential is held very negative (generally below −80 mV) for severalmilliseconds. The time constants and threshold potentials fortransitions between these three states vary greatly between channelsubtypes.

[0295] During these experiments, the response will be compared for cellswith active channels, and for cells in which the channels arepharmacologically blocked. If a suitable pharmacological agent is notavailable, the blocked state can be emulated with an un-transfected cellline. The optimal stimulus parameters will yield the smallestcoefficient of variation of the difference in signals of the two cellpopulations.

[0296] (a) Assays for Voltage-Dependent Sodium Channels in anInactivated State

[0297] Preferred cells include those with resting transmembranepotentials above the activation threshold for the ion channel ofinterest, and in which there are no other ion channels expressed. Cellsmeeting these criteria include CHL and LTK(−) cells. After choosing atarget ion channel, cells are transfected and clones are selected asdescribed in section III. Alternatively, a cell line that endogenouslyexpresses the channel of interest, and low levels of other channels,could be used. For example, the CHO-K1 cell line expresses avoltage-gated sodium channel, and very low levels of other ion channels.Cells are plated into multiwell microtiter plates, cultured, and stainedwith voltage-sensitive dyes as described in section IV prior toinitiating electrical stimulation. Initial experiments are typicallycarried out in a 96-well multiwell plate, with an equal number of cellsin each well. Generally columns of eight wells are simultaneouslystimulated under identical conditions to provide statisticallysignificant data on the variation in cellular response.

[0298] An optimal electrical stimulation protocol should hyperpolarizepart of the plasma membrane of the majority of the cells long enough torelease the sodium channels from inactivation, prior to providing anactivating depolarization, without electroporating or killing the cells.Typically this requires sustained transmembrane potentials of around −60to −80 mV for periods ranging from about 0.5 to about 20 ms to becreated within the cell.

[0299] A preferred stimulation protocol that achieves this effect isbiphasic, so that ion channels present on both the extreme edges of thecells are released from inactivation as the biphasic waveform reversespolarity. Typically one would start out with initial conditions using abiphasic square wave kernel of 5 msec per phase and an amplitude of 25V/cm. The kernel would be repeated at a regular rate of about 20 Hz fora total train duration of about three seconds. One would then optimizethe pulse amplitude (up to a maximum of about 60 V/cm), duration (in therange of 0.1 to 50 ms), and then frequency (in the range of 0 to 1 kHz).If necessary changes in the pulse shape could also be explored todetermine if these resulted in more efficient electrical stimulation.The optimal stimulus parameters will yield the maximum cellularstimulation (compared to cells with the channel blocked, or not present)with smallest coefficient of variation of the signal among the differenttest wells, at the lowest electric field strength, and at the lowestduty cycle for passage of current through the electrodes. After aparticular set of parameters is chosen, a titration of stainingconcentrations for the voltage sensor dye(s) should be performed asdescribed above, to further optimize the signal size and coefficient ofvariation of the responses. These procedures (dye concentrations,electric field strength, and stimulus duration and frequency) can beiterated to further optimize the signal.

[0300] (b) Assays for Sodium Channels Normally in the Resting State

[0301] Preferred cells include those with resting transmembranepotentials below the activation threshold for the ion channel ofinterest, and in which the expression of other ion channels is largelyconfined to a few characterized ion channel types. Cells of this typeinclude HEK-293 and RBL cells as well as F11 and HL5 cells. Afterchoosing a target ion channel, cells are transfected with the ionchannel of interest and clones are selected as described above.Alternatively, as in the case of F11 and HL5 cells, endogenous sodiumchannels can be used. After selection and characterization, cell clonesare plated into multiwell microtiter plates and stained withvoltage-sensitive dyes as described above. As previously, initialexperiments are typically carried out in a 96-well multiwell plate, withan equal number of cells in each well. Generally columns of eight wellsare simultaneously stimulated under identical conditions to providestatistically significant data on the variation in cellular response.

[0302] A number of assay approaches are possible depending on theexpression level of the sodium channel of interest in the cell. For highlevels of voltage-dependent sodium channel expression, the sodiumcurrent can be large enough to create a large transmembrane potentialchange after a single channel activation/inactivation sequence. In thesecases small positive perturbations in the transmembrane potentialcreated via electrical stimulation can be sufficient to activate enoughsodium channels that the subsequent sodium ion entry depolarizes theentire cell thereby activating all the sodium channels. The stimulusfield should typically be applied for a time long enough to activate thechannels, but not so long as to interfere with the subsequent ion flux.After the cell transmembrane potential has re-polarized, the stimulationprocedure can be repeated. Subsequent stimulation events can beidentical to the first, or varied to examine time-dependent propertiesof the channels.

[0303] Typically one would start out with initial conditions using abiphasic square wave kernel of 500 μs per phase and an amplitude of 10V/cm. One would then optimize the pulse amplitude (between 5 and 60V/cm) and duration (between 0.1 and 1 ms). If necessary changes in thepulse shape could also be explored to determine if these resulted inmore efficient electrical stimulation. The optimal stimulus parameterswill yield the maximum cellular stimulation with smallest coefficient ofvariation of the signal among the different test wells, at the lowestelectric field strength, and at the lowest duty cycle for passage ofcurrent through the electrodes. After a particular set of parameters ischosen, a titration of staining concentrations for the voltage sensordye(s) should be performed as described above, to further optimize thesignal size and coefficient of variation of the responses. Theseprocedures (dye concentrations, electric field strength, and stimulusduration and frequency) can be iterated to further optimize the signal.Often it will be necessary to use cells whose expression of sodiumchannels is too low to give acceptable signal sizes from single stimuli.It may also be desirable to maintain a large signal over an extendedperiod of time. In these cases, the cells can be given pulse trains asdescribed for channels held above the activation potential. Withbiphasic stimulus pulses, the sodium channels can be activatedindependent of the starting transmembrane potential. By keeping theinter-pulse interval shorter than the membrane time constant, eachstimulus will drive current into the cell until an equilibrium betweeninward and outward currents is established. This voltage deviation willbe maintained as long as the stimulus train continues.

[0304] The stimulation protocols in this case are essentially the sameas described for cells whose resting potential is above the inactivationthreshold. In general, a series of initial experiments are conductedusing a biphasic square wave kernel repeated at a regular rate for afixed train duration. The pulse duration varies from about 1 μs to about1 s, and more preferably from about 100 μs to about 20 ms. The pulseamplitude varies from 0 V/cm to about 60 V/cm, and more preferably from10 V/cm to 50 V/cm. The frequency of stimulation varies between 0 Hz(i.e. a single pulse) and 100 kHz, and more preferably from 0 Hz toabout 1 kHz. The pulse train varies between 0 s (i.e. a single pulse)and about 100 s, and more preferably between 0 s and 10 s. The optimalstimulus parameters will yield the maximum transmembrane potentialchanges (compared to cells with the channel blocked, or not present) andsmallest coefficient of variation of the signal among the test wells, atthe lowest electric field strength. After a particular set of parametersis chosen, a titration of staining concentrations for the voltage sensordye(s) is typically performed as described above to further optimize thesignal size and coefficient of variation of the responses. Theseprocedures (dye concentrations, electric field strength, and stimulusduration and frequency) can be iterated to further optimize the signal.

[0305] ii. Potassium Channels

[0306] Voltage-dependent potassium channels repolarize nerve and musclecells after action potential depolarization. They also play importantregulatory roles in neural, muscular, secretory, and excretory systems.Most cells actively maintain a high intracellular potassiumconcentration, so that the reversal transmembrane potential forpotassium is around −90 mV. Potassium typically flows out of the cell,so that opening more potassium-selective channels tends to drive thetransmembrane potential more negative, in contrast to sodium channelopening that typically drives the transmembrane potential more positive.

[0307] A summary of the numerous potassium sub-types is presented inTable 2 below. TABLE 2 Potassium Channel Sub-type Summary AccessionChannel Type Sub-type/Alternate names Number Reference ATP regulatedrKir1.1 (ROMK1) (rat) U12541 U.S. Pat. No. 5,356,775 hKir1.1 (ROMK1)(human) U.S. Pat. No. 5,882,873 Kir1.2 U73191 Kir1.3 U73193 II. β-cellU.S. Pat. No. 5,744,594 III. hβIR U.S. Pat. No. 5,917,027 IV.HuK_(ATP)-1 EP 0 768 379 A1 Constitutively Active Kir2.1 (IRK1) U12507U.S. Pat. No. 5,492,825 U.S. Pat. No. 5,670,335 Kir2.2 X78461 Kir2.3U07364 G-protein Regulated Kir3.1 (GIK1, KGA) U01071 U.S. Pat. No.5,728,535 Kir3.2 U11859 U.S. Pat. No. 5,734,021 Kir3.3 U11869 U.S. Pat.No. 5,744,324 Kir3.4 (CIR) X83584 U.S. Pat. No. 5,747,278 Kir4.1 (BIR10)X83585 Kir5.1 (BIR9) X83581 Kir6.1 D42145 Kir6.2 D5081 Kir7.1 EP 0 922763 A1 Voltage Regulated KCNA1 hKv1.1 (RCK1, RBK1, MBK1, MK1, LO2750HuK1) KCNA2 Kv1.2 (RBK2, RBK5, NGK1, HuKIV) KCNA3 Kv1.3 (KV3, RGK5,HuKIII, HPCN3, KCNA4 Kv1.4 (RCK4, RHK1, HuKII) KCNA5 Kv1.5 (KV1, HPCN1,HK2) KCNA6 Kv1.6 (KV2, RCK2, HBK2) KCNA7 Kv1.7 (MK6, RK6, HaK6) U.S.Pat. No. 5,559,009 Kv2 (Shab) KCNB1 Kv2.1 (DRK1, mShab) M64228 KCNB2Kv2.2 (CDRK1) K channel 2 U.S. Pat. No. 5,710,019 Kv3 (Shaw) KCNC1 Kv3.1(NGK2) KCNC2 Kv3.2 (RKShIIIA) KCNC3 Kv3.3 (KShIIID) X60796 KCNC4 Kv3.4(Raw3) Kv4 (Shal) KCND1 Kv4.1 (mShal, KShIVA) M64226 KCND2 Kv4.2 (RK5,Rat Shal 1) KCND3 Kv4.3 (KShIVB) hKv5.1 (IK8) WO 99/41372 Kv6.1 (K13)Kv7 Kv8.1 Kv9 Delayed Rectifier KvLQT1 AF000571 U.S. Pat. No. 5,599,673HERG (erg) U04270 PCT WO99/20760 Calcium regulated Ca²⁺ Regulated BigBKCa (hSLO) U11717 HBKb3 (β-subunit) PCT WO99/42575 Maxi-K U.S. Pat. No.5,776,734 U.S. Pat. No. 5,637,470 Ca²⁺ Regulated-small KCNN1 SKCa1U69883 KCNN2 SKCa2 U69882 KCNN3 SKCa3 U69884 KCNN4 SKCa4 (IKCa1) MuscleNerve 1999 22(6) 742-50 TWIK1 U33632

[0308] Potassium channels show enormous diversity in terms of activationand inactivation time constants and voltage dependencies. In general,voltage-dependent potassium channels show voltage dependence similar tosodium channels, being closed at very negative potentials and openingabove a certain threshold. Potassium channels may have multiple restingstates, multiple inactivated states, and typically a single activatedstate. Unlike voltage-dependent sodium channels, transitions are allowedbetween most states. These transitions are activation (moving from aresting to the open state), deactivation (moving from the open state toa resting state), inactivation (moving from a resting or open state toan inactivated state), release from inactivation (moving from aninactivated state to a resting state), and flickering (moving from aninactivated state to the open state). There is a great diversity in thethresholds of the transitions, and in the voltage dependencies of thetransition rates. Activation time constants range from 0.1 to 1000 mswith threshold activation potentials from −80 to +20 mV. Inactivationtime constants range from 0.1 to infinity (i.e. no inactivation) withthreshold potentials from −60 to 0 mV. Time constants for release frominactivation range from 0.5 ms to 100 ms with threshold potentials from−70 to 0 mV.

[0309] Stimulus protocols necessary to obtain measurablechannel-dependent signals are somewhat dependent upon the specificproperties of the channel in question. Because of the diversity inparameters in voltage-dependent potassium channels, the optimization ofan electrical stimulation protocol may take several iterations.

[0310] During these experiments, the response will be compared for cellswith active channels, and for cells in which the channels arepharmacologically blocked. If a suitable pharmacological agent is notavailable, the blocked state can be emulated with an un-transfected cellline. The optimal stimulus parameters will yield the smallestcoefficient of variation of the difference in signals of the two cellpopulations.

[0311] (a) Assays Using Direct Stimulation of the Potassium Channel

[0312] (i) Voltage Regulated Potassium Channels

[0313] Because potassium channels generate outward currents, activatingthe channels causes negative transmembrane potential changes. Underphysiological conditions, the reversal potential for potassium is around−90 mV. Because cells expressing only a voltage-dependent potassiumchannel generally have resting potentials near the activation threshold,direct stimulation should work for those voltage-dependent potassiumchannels which have activation thresholds above about −50 mV. Whilesmall negative deflections in the transmembrane potential (less than 40mV change) can be reliably detected using the FRET voltage-sensitivedyes, it is often preferable to perform high-throughput screens withlarger signals.

[0314] Preferred cell types include those cells that express a minimallevel of other ion channels, such as CHO-K1, CHL, and LTK(−). Thetransfection and selection of clones expressing ion channels of interestwill generally be performed as described above for sodium ion channelsnormally in the resting state. Alternatively, a cell line whichendogenously expresses the channel of interest could be used. Thelabeling and measurement of cells with transmembrane potential dyes willgenerally be performed as described for sodium ion channels normally inthe resting state.

[0315] The stimulation protocol will advantageously depolarize part ofthe plasma membrane long enough to activate the voltage-dependentpotassium channels. Unlike the case for voltage-dependent sodiumchannels, voltage-dependent potassium channels will typically passcurrent during the depolarizing phase of the stimulus pulse. On the sideof the cell where the transmembrane potential is driven in a negativedirection, the potassium channels release from inactivation (if thechannel in question experiences voltage-dependent inactivation). On theside of the cell where the transmembrane potential is driven in apositive direction, potassium channels activate and pass outwardcurrent. Thus, the stimulus pulse duration should not greatly exceed theinactivation time. The potassium current tends to drive the averagetransmembrane potential negative of the resting potential. After thestimulus pulse, the transmembrane potential will exponentially relax tothe resting potential. By repeating the stimulus after a time shorterthan the membrane time constant, the average cell membrane can be drivenfurther negative. Using a train of stimuli, a large and sustained signalcan be obtained.

[0316] A preferred stimulation protocol that achieves this effect isbiphasic, so that ion channels present on both the extreme edges of thecells can participate in enabling potassium ion movement. Typically onewould start out with initial conditions using a biphasic square wavekernel of 5 msec per phase and an amplitude of 25 V/cm. The kernel wouldbe repeated at a regular rate of about 20 Hz for a total train durationof about three seconds. One would then optimize the pulse amplitude,duration, and then frequency. If necessary changes in the pulse shapecould also be explored to determine if these resulted in more efficientelectrical stimulation. The optimal stimulus parameters will yield themaximum average transmembrane potential change (compared to cells withthe channel blocked, or not present) with smallest coefficient ofvariation of the signal among the different test wells, at the lowestelectric field strength, and at the lowest duty cycle for passage ofcurrent through the electrodes. After a particular set of parameters ischosen, a titration of staining concentrations for the voltage sensordye(s) should be performed as described above, to further optimize thesignal size and coefficient of variation of the responses. Theseprocedures (dye concentrations, electric field strength, and stimulusduration and frequency) can be iterated to further optimize the signal.

[0317] (b) Inward-Rectifier Potassium Channels

[0318] Contrary to its name, the function of the inward rectifierchannel is not to allow potassium into the cell. Inward flow ofpotassium can only occur (1) when the transmembrane potential fallsbelow the potassium equilibrium potentials, or (2) if the extracellularpotassium concentration rises. Neither situation normally occurs,because (1) under normal physiological conditions, since potassium isthe ion with the most negative reversal potential, no ionic current candrive the potential more negative than the potassium reversal potential,and (2) except under pathological conditions, the extracellularpotassium concentration is tightly controlled. However, using electricalstimulation, parts of the cell membrane can be driven below V_(K),promoting potassium ion entry into the cell. This will cause a netpositive transmembrane potential change and can be detected as apositive signal. To develop and optimize an assay for blockers of theinward rectifier, one could therefore follow the following procedure.

[0319] Preferred cell types include those cells that express a minimallevel of other ion channels, such as CHO-K1, CHL, and LTK(−). Thetransfection and selection of clones expressing ion channels of interestwill generally be performed as described above for sodium ion channelsnormally in the resting state. Alternatively, a cell line whichendogenously expresses the channel of interest could be used. Thelabeling and measurement of cells with transmembrane potential dyes willgenerally be performed as described for sodium ion channels normally inthe resting state.

[0320] A preferred stimulation protocol uses a biphasic kernel, so thation channels present on both the extreme edges of the cells participate.Typically one would start out with initial conditions using a biphasicsquare wave kernel of 5 msec per phase and an amplitude of 25 V/cm. Thekernel would be repeated at a regular rate of about 20 Hz for a totaltrain duration of about three seconds. One would then optimize the pulseamplitude, duration, and then frequency. If necessary changes in thepulse shape could also be explored to determine if these resulted inmore efficient electrical stimulation. The optimal stimulus parameterswill yield the maximum cellular stimulation (compared to cells with thechannel blocked, or not present) with the smallest coefficient ofvariation of the signal among the different test wells, at the lowestelectric field strength, and at the lowest duty cycle for passage ofcurrent through the electrodes. After a particular set of parameters ischosen, a titration of staining concentrations for the voltage sensordye(s) should be performed as described above, to further optimize thesignal size and coefficient of variation of the responses. Theseprocedures (dye concentrations, electric field strength, and stimulusduration and frequency) can be iterated to further optimize the signal.

[0321] (c) Assays Using a Voltage-Dependent Sodium Counter-Channel

[0322] This method involves the use of a cell line expressing thevoltage-dependent potassium channel of interest and which also expressesa voltage-dependent sodium channel. In this method the approach is touse electrical stimulation protocols designed to specifically activatethe voltage dependent sodium channel. In this case electricalstimulation causes sodium ions to enter the cell, causing a positivevoltage change. The presence of the potassium channel of interest willtend to suppress the positive response of the sodium channel by allowingpotassium ions to leave the cell. The assay takes advantage of theabsence of outward current when a test chemical blocks the potassiumchannel, thereby restoring the large positive voltage response normallyinduced by activation of the sodium channels. The optimization of thebalance of currents is important in this method to ensure that the assayis sensitive to potassium channel blockade. If the sodium current is toosmall relative to the potassium current, the dose-response curve for thepotassium channel blocker will be shifted towards higher concentrations.For example, in the extreme case where the potassium current is 100times larger than the sodium current, 99% of the potassium channelswould have to be blocked in order to get a 50% response from the sodiumchannels.

[0323] Because this method involves driving a voltage-dependent sodiumchannel with repetitive pulses, the protocol development is essentiallythe same as described above for voltage-activated sodium channels in aninactivated state. Typically one would start out with initial conditionsusing a biphasic square wave kernel of 5 msec per phase and an amplitudeof 25 V/cm. The kernel would be repeated at a regular rate of about 20Hz for a total train duration of about three seconds. One would thenoptimize the pulse amplitude, duration, and then frequency. If necessarychanges in the pulse shape could also be explored to determine if theseresulted in more efficient electrical stimulation.

[0324] The optimal stimulus parameters will yield the maximum cellularstimulation (compared to cells with the channel blocked, or not present)with smallest coefficient of variation of the signal among the differenttest wells, at the lowest electric field strength, and at the lowestduty cycle for passage of current through the electrodes. After aparticular set of parameters is chosen, a titration of stainingconcentrations for the voltage sensor dye(s) should be performed asdescribed above, to further optimize the signal size and coefficient ofvariation of the responses. These procedures (dye concentrations,electric field strength, and stimulus duration and frequency) can beiterated to further optimize the signal.

[0325] In this assay format, there will ideally be no (or a very small)response to stimulation in the absence of channel block, because thepotassium current will counteract the sodium current. Therefore, tooptimize the stimulus conditions, it will be necessary to compareresponses with and without the activity of the potassium channel.Ideally, this will be accomplished using a selective blocker of thepotassium channel. In those cases where such a blocker is yet unknown,it will be possible to use the cell line containing only the sodiumcounter-channel.

[0326] Because this assay format involves two ion channels, modulatorsof either channel will affect the voltage response. In this case, a hit(a blocker of the potassium channel) will restore the voltage response.The screening format automatically ignores compounds which block onlythe sodium channel. However, stimulation of the cells in the presence ofcompounds which block both channels will also result in no voltagedeflection, suggesting that the compound is inactive. Because compoundsof this type may be of interest, a method to unmask them is alsoavailable. By performing the identical compound screen using the parentcell line, which contains the sodium channel but not the potassiumchannel, blockers of the sodium channel can be found. Compounds whichare found to block the sodium channel can then be tested separately tofind if they have activity against the potassium channel.

[0327] ii. Assay of Calcium Channels

[0328] Calcium channels are generally found in many cells where, amongother functions, they play important roles in signal transduction. Inexcitable cells, intracellular calcium supplies a maintained inwardcurrent for long depolarizing responses and serves as the link betweendepolarization and other intracellular signal transduction mechanisms.Like voltage-gated sodium channels, voltage-gated calcium channels havemultiple resting, activated, and inactivated states.

[0329] Multiple types of calcium channels have been identified inmammalian cells from various tissues, including skeletal muscle, cardiacmuscle, lung, smooth muscle and brain, [see, e.g., Bean, B. P. (1989)Ann. Rev. Physiol. 51:367-384 and Hess, P. (1990) Ann. Rev. Neurosci.56:337]. The different types of calcium channels have been broadlycategorized into four classes, L-, T-, N-, and P-type, distinguished bycurrent kinetics, holding potential sensitivity and sensitivity tocalcium channel agonists and antagonists. Four subtypes of neuronalvoltage-dependent calcium channels have been proposed (Swandulla, D. etal., Trends in Neuroscience 14:46, 1991).

[0330] The cDNA and corresponding amino acid sequences of the α1, α2, β,and γ subunits of the rabbit skeletal muscle calcium channel have beendetermined [see, Tanabe et al. (1987) Nature 328:313-318; Ruth et al.(1989) Science 245:1115-1118; and U.S. Pat. No. 5,386,025]. In addition,the cDNA and corresponding amino acid sequences of α1 subunits of rabbitcardiac muscle [Mikami, A. et al. (1989) Nature 340:230-233] and lung[Biel, M. (1990) FEBS Letters 269:409-412] calcium channels have beendetermined. In addition, cDNA clones encoding a rabbit brain calciumchannel (designated the BI channel) have been isolated [Mori, Y. et al.(1991) Nature 350:398-402].

[0331] Partial cDNA clones encoding portions of several differentsubtypes, referred to as rat brain class A, B, C and D, of the calciumchannel α1 subunit have been isolated from rat brain cDNA libraries[Snutch, T. et al. (1990) Proc. Natl. Acad. Sci. USA 87:3391-3395]. Morerecently full-length rat brain class A [Starr, T. et al. (1991) Proc.Natl. Acad. Sci. USA 88:5621-5625] and class C [Snutch, T. et al. (1991)Neuron 7:45-57] cDNA clones have been isolated. Although the amino acidsequence encoded by the rat brain class C DNA is approximately 95%identical to that encoded by the rabbit cardiac muscle calcium channelα1 subunit-encoding DNA, the amino acid sequence encoded by the ratbrain class A DNA shares only 33% sequence identity with the amino acidsequence encoded by the rabbit skeletal or cardiac muscle α1subunit-encoding DNA. A cDNA clone encoding another rat brain calciumchannel α1 subunit has also been obtained [Hui, A. et al. (1991) Neuron7:35-44]. The amino acid sequence encoded by this clone is approximately70% homologous to the proteins encoded by the rabbit skeletal andcardiac muscle calcium channel DNA. A cDNA clone closely related to therat brain class C α1 subunit-encoding cDNA and sequences of partial cDNAclones closely related to other partial cDNA clones encoding apparentlydifferent calcium channel α1 subunits have also been isolated [seeSnutch, T. et al. (1991) Neuron 7:45-57; Perez-Reyes, E. et al. (1990)J. Biol. Chem. 265:20430; and Hui, A. et al. (1991) Neuron 7:35-44].

[0332] For known calcium channels that have been characterized,activation time constants range from 0.1 to 10 ms with thresholdpotentials from −80 to −20 mV. Inactivation time constants range from0.1 to ∞ (i.e. no inactivation) with threshold potentials from −60 to−20 mV. Time constants for release from inactivation range from 0.5 msto 100 ms with threshold potentials from −70 to −40 mV.

[0333] Choice of cell line and induction of voltage-dependent calciumcurrents are performed using the general guidelines and approachesdiscussed above for sodium channels. Preferred cell types include thosecells that express a minimal level of other ion channels, such asCHO-K1, CHL, and LTK(−). The transfection and selection of clonesexpressing ion channels of interest will generally be performed asdescribed above for sodium ion channels normally in the resting state.Alternatively, a cell line which endogenously expresses the channel ofinterest could be used. The labeling and measurement of cells withtransmembrane potential dyes will generally be performed as describedfor sodium ion channels normally in the resting state. Alternatively,the cells can be loaded with calcium-sensitive fluorescent dyes such asCalcium Green, fluo3-AM, or indo-1.

[0334] In cells with low background currents, strong inward calciumcurrents can be generated by driving portions of the membrane negativeenough to release the channels from inactivation. Then by reversing orreleasing the external electric field, the channels are exposed topotentials which activate the channels and permit calcium current toflow into the cell. The reversal potential for calcium in most cells isgenerally +60 to +100 mV, so large voltage changes due to calcium influxare possible. We can use either membrane-bound voltage-sensitive dyes orintracellular calcium dyes to monitor the activity of the cells. Due tothe similarity in properties of calcium and sodium channels, the samegeneral assay optimization procedures outlined above for sodium channelswill apply to calcium channels.

[0335] Typically one would start out with initial conditions using abiphasic square wave kernel of 5 msec per phase and an amplitude of 25V/cm. The kernel would be repeated at a regular rate of about 20 Hz fora total train duration of about three seconds. One would then optimizethe pulse amplitude, duration, and then frequency. If necessary changesin the pulse shape could also be explored to determine if these resultedin more efficient electrical stimulation. The optimal stimulusparameters will yield the maximum cellular stimulation (compared tocells with the channel blocked, or not present) with the smallestcoefficient of variation of the signal among the different test wells,at the lowest electric field strength, and at the lowest duty cycle forpassage of current through the electrodes. After a particular set ofparameters is chosen, a titration of staining concentrations for thevoltage sensor dye(s) should be performed as described above, to furtheroptimize the signal size and coefficient of variation of the responses.These procedures (dye concentrations, electric field strength, andstimulus duration and frequency) can be iterated to further optimize thesignal.

[0336] During these experiments, the response will be compared for cellswith active channels, and for cells in which the channels arepharmacologically blocked. If a suitable pharmacological agent is notavailable, the blocked state can be emulated with an un-transfected cellline. The optimal stimulus parameters will yield the smallestcoefficient of variation of the difference in signals of the two cellpopulations.

[0337] iii. Assay of Voltage-Dependent Chloride Channels

[0338] Chloride channels are found in the plasma membranes of virtuallyevery cell in the body. Chloride channels mediate a variety of cellularfunctions including regulation of transmembrane potentials andabsorption and secretion of ions across epithelial membranes. Whenpresent in intracellular membranes of the Golgi apparatus and endocyticvesicles, chloride channels also regulate organelle pH. For a review,see Greger, R. (1988) Annu. Rev. Physiol. 50:111-122.

[0339] Three distinct classes of chloride channels are apparent based ontheir type of regulation and structural conformation, Table 3. The firstclass includes the GABA and Glycine receptor super families, the secondclass includes the CFTR (Cystic fibrosis Transmembrane ConductanceRegulator) and the third class includes the voltage regulated chloridechannels. TABLE 3 iv. Chloride Channel Sub-type Summary Tissue ChannelType Sub-type Distribution Reference Ligand gated GABA_(A) CNS & PNSSynapse 21, 189-274 Receptor (1995) family Glycine CNS & PNS TrendsNeurosci. Receptor 14, 458-461 family (1991) cAMP regulated CFTREpithelial Science 245, tissues 1066-1073 (1989) Voltage regulated ClC-1Skeletal Nature 354, 301-304 Muscle (1991) ClC-2 Ubiquitous Nature 356,57-60 (1992) ClC-Ka Kidney J. Biol. Chem. $$ ClC-Kb Kidney P.N.A.S. 91,6943-6947 (1994) ClC-3 Broad, e.g. Neuron 12, 597-604 Kidney & (1994)Brain ClC-4 Broad, e.g. Hum. Mol. Genet. Kidney & 3, 547-552 (1994)Brain ClC-5 Mainly Kidney J. Biol. Chem. 270, 31172-31177 (1995) ClC-6Ubiquitous FEBS. Lett. 377, 15-20 (1995) ClC-7 Ubiquitous FEBS. Lett.377, 15-20 (1995)

[0340] In contrast to ions like sodium and especially calcium, theelectrochemical gradient of chloride across the plasma membrane isgenerally not far from equilibrium. Thus, at the resting potential ofcells, the opening of chloride channels will not lead to largeexcursions of the plasma membrane voltage or dramatic changes inintracellular chloride concentrations. Because electrical stimulationtypically generates symmetrical voltage changes across the cellmembrane, no net chloride flux can be generated unless the conductivityof the channel is non-linear. For a linear leak conductance, a uniformelectric field will drive chloride into the cell on one side and out ofthe cell on the other side.

[0341] Direct electrical stimulation of chloride channels which havenon-linear conductance curves (rectifiers) or voltage-activated gatingcan generate net ion fluxes, which in turn will cause detectabletransmembrane potential changes. Depending upon the voltage dependenceof the conductance and gating, the transmembrane potential change can beeither positive or negative. For typical chloride channels (thatactivate at elevated potentials and close at more negative potentials)and for outward rectifiers, chloride will flow into the cell and drivethe transmembrane potential negative. For inward rectifiers, chloridewill be driven out of the cell and the transmembrane potential will bedriven positive.

[0342] Due to the small difference between the chloride reversalpotential and the resting transmembrane potential, direct stimulation ofa voltage-gated chloride channel may result in insufficienttransmembrane potential changes. Assays for these ion channels can thenbe developed using co-expression and electrical stimulation of a sodiumor potassium counter-channel in order to produce an inward or outwardcurrent. Presence or absence of the chloride current can then bedetermined by the absence or presence of a transmembrane potentialchange when the counter-channel is electrically stimulated. Preferredcell types include those cells that express a minimal level of other ionchannels, such as CHO-K1, CHL, and LTK (−). The transfection andselection of clones expressing ion channels of interest will generallybe performed as described above for sodium ion channels normally in theresting state. Alternatively, a cell line which endogenously expressesthe channel of interest (or the counter-channel) could be used. Thelabeling and measurement of cells with transmembrane potential dyes willgenerally be performed as described for sodium ion channels normally inthe resting state.

[0343] Typically one would start out with initial conditions using abiphasic square wave kernel of 5 msec per phase and amplitude of 25V/cm. The kernel would be repeated at a regular rate of about 20 Hz fora total train duration of about three seconds. One would then optimizethe pulse amplitude, duration, and then frequency. If necessary changesin the pulse shape could also be explored to determine if these resultedin more efficient electrical stimulation. The optimal stimulusparameters will yield the maximum cellular stimulation (compared tocells with the channel blocked, or not present) with smallestcoefficient of variation of the signal among the different test wells,at the lowest electric field strength, and at the lowest duty cycle forpassage of current through the electrodes. After a particular set ofparameters is chosen, a titration of staining concentrations for thevoltage sensor dye(s) should be performed as described above, to furtheroptimize the signal size and coefficient of variation of the responses.These procedures (dye concentrations, electric field strength, andstimulus duration and frequency) can be iterated to further optimize thesignal.

[0344] During these experiments, the response will be compared for cellswith active channels, and for cells in which the channels arepharmacologically blocked. If a suitable pharmacological agent is notavailable, the blocked state can be emulated with an un-transfected cellline. The optimal stimulus parameters will yield the smallestcoefficient of variation of the difference in signals of the two cellpopulations.

[0345] v. Assay of Ligand Dependent Channels

[0346] The ligand-dependent ion channel family is large and diverse.Ligand-dependent ion channels open in response to the binding ofspecific molecules. They typically mediate fast synaptic transmissionbetween neurons, and from neurons to muscle cells. They also mediateslow synaptic transmission and control a variety of regulatorymechanisms. Ligand-gated ion channels are generally onlycharge-selective; that is, they permit the flow of a range of eitheranions or cations but have little specificity. They have enormousvariation in their activation, deactivation, and desensitizationkinetics, all of which can vary from submillisecond to second timeconstants.

[0347] When the ligand binds to the receptor of the channel, the channelundergoes one or more conformational changes to activate the channel. Ifthe ligand is removed from the bathing saline, the bound ligandsdissociate and the channel closes. If the ligand remains in the bathingsaline, some channels desensitize by retaining the ligand but movinginto a different conformational state in which the channel is closed.Equilibrium distributions between the activated, deactivated, anddesensitized states vary greatly among channels.

[0348] In current assay formats, the transmembrane potential of thecells is monitored during an addition of ligand. The sudden increase inconductance when the channel opens drives the transmembrane potentialtowards a new reversal potential. Unfortunately, for many ligand-gatedchannels, the new reversal potential is usually within 15 mV of theresting potential. This small change is sufficient to use for signalingwithin cells, but it makes pharmacological assays difficult.

[0349] In an electrical stimulation assay for ligand-gated ion channels,one approach is to co-express a voltage-gated sodium counter channelwith the ligand gated ion channel of interest. This approach allows usto modulate the transmembrane potential via electrical stimulation. Ifthe test compounds are added to the cells during or prior to electricalstimulation, the method enables an analysis of whether the ligand gatedchannel is open or closed. If the ligand-gated channels are open, thehigh resting conductance of the cell will suppress the voltage responseto electrical stimulation. If, however, the ligand-gated channels areblocked, the cells will have a large response to electrical stimulation.The large amount of flexibility in electrical stimulation parametersshould allow us to assay for a large range in resting conductances. Thisis important in the case of ligand-gated channels, because the restingconductance in the presence of ligand is very sensitive to theequilibrium desensitization. Accounting for desensitization andvariations in channel expression, we may have resting membraneresistances ranging anywhere from 10 MΩ to 10 GΩ. With rat brain typeIIa sodium channels as the counter channel, we can cover this entirerange. It should also be possible to screen for both agonists andantagonists. By choosing stimulation parameters such that the responseis half-size, agonists will reduce the response while antagonists willincrease it. Better screening windows may be obtained by stimulating athigher (agonist assay) or lower (antagonist assay) frequencies. Notethat modulators of the channel conductance, open time, desensitization,and deactivation will all be detected.

[0350] Preferred cell types include those cells that express a minimallevel of other ion channels, such as CHO-K1, CHL, and LTK (−). Thetransfection and selection of clones expressing ion channels of interestwill generally be performed as described above for sodium ion channelsnormally in the resting state. Alternatively, a cell line whichendogenously expresses the channel of interest (or the counter-channel)could be used. The labeling and measurement of cells with transmembranepotential dyes will generally be performed as described for sodium ionchannels normally in the resting state.

[0351] Typically one would start out with initial conditions using abiphasic square wave kernel of 5 msec per phase and amplitude of 25V/cm. The kernel would be repeated at a regular rate of about 20 Hz fora total train duration of about three seconds. One would then optimizethe pulse amplitude, duration, and then frequency. If necessary changesin the pulse shape could also be explored to determine if these resultedin more efficient electrical stimulation. The optimal stimulusparameters will yield the maximum cellular stimulation (compared tocells with the ligand-gated channel blocked, or not present) withsmallest coefficient of variation of the signal among the different testwells, at the lowest electric field strength, and at the lowest dutycycle for passage of current through the electrodes. After a particularset of parameters is chosen, a titration of staining concentrations forthe voltage sensor dye(s) should be performed as described above, tofurther optimize the signal size and coefficient of variation of theresponses. These procedures (dye concentrations, electric fieldstrength, and stimulus duration and frequency) can be iterated tofurther optimize the signal.

[0352] During these experiments, the response will be compared for cellswith active channels, and for cells in which the channels arepharmacologically blocked. If a suitable pharmacological agent is notavailable, the blocked state can be emulated with an un-transfected cellline. The optimal stimulus parameters will yield the smallestcoefficient of variation of the difference in signals of the two cellpopulations.

[0353] vi. Assay of Passive Channels

[0354] Many channels have slow or no voltage-activated conductancechanges. Prime examples are the some of the channels implicated incystic fibrosis, particularly the cystic fibrosis transmembraneregulator (CFTR, a chloride channel), the epithelial sodium channel(ENaC) and 4 TM potassium channel family members (Wang et al. Ann. N. Y.Acad. Sci. 868: 286-303, 1999). A small molecule which acts as anagonist for either of these channels would be a candidate for a drugwhich alleviates cystic fibrosis. Currently, there is no convenientworkable high throughput screening method for channels of this type.

[0355] The proposed assay format for ion channel targets of this typeinvolves a cell expressing the leak channel of interest in a cell whichalso expresses a voltage-dependent sodium channel. The channel ofinterest is cloned into a cell with a voltage-dependent sodium channel.The presence of the passive current will suppress. the positive responseof the sodium channel when the cells are stimulated. Blocking thepassive channel will restore the large positive voltage response.Optimization of the balance of currents will be important in thismethod. Wild-type CHO cell may be useful for this purpose, although acell with larger sodium currents (either endogenous or engineered) wouldbe preferable. If the sodium current is too small relative to thepotassium current, the dose-response curve for the passive channelblocker will be shifted towards higher concentrations. For example, inthe extreme case where the passive current is 100 times larger than thesodium current, 99% of the passive channels would have to be blocked inorder to get a 50% response from the sodium channels.

[0356] Preferred cell types include those cells that express a minimallevel of other ion channels, such as CHO-K1, CHL, and LTK (−). Thetransfection and selection of clones expressing ion channels of interestwill generally be performed as described above for sodium ion channelsnormally in the resting state. Alternatively, a cell line whichendogenously expresses the channel of interest (or the counter-channel)could be used. The labeling and measurement of cells with transmembranepotential dyes will generally be performed as described for sodium ionchannels normally in the resting state.

[0357] A preferred stimulation protocol uses a biphasic kernel. Ingeneral, a series of initial experiments are conducted using a biphasicsquare wave kernel repeated at a regular rate for a fixed trainduration. The pulse duration varies from about 1 μs to about 1 s, andmore preferably from about 100 μs to about 20 ms. The pulse amplitudevaries from 0 V/cm to about 60 V/cm, and more preferably from 10 V/cm to50 V/cm. The frequency of stimulation varies between 0 Hz (i.e. a singlepulse) and 100 kHz, and more preferably from 0 Hz to about 1 kHz. Thepulse train varies between 0 s (i.e. a single pulse) and about 100 s,and more preferably between 0 s and 10 s.

[0358] Typically one would start out with initial conditions using abiphasic square wave kernel of 5 msec per phase and an amplitude of 25V/cm. The kernel would be repeated at a regular rate of about 20 Hz fora total train duration of about three seconds. One would then optimizethe pulse amplitude, duration, and then frequency. If necessary changesin the pulse shape could also be explored to determine if these resultedin more efficient electrical stimulation. The optimal stimulusparameters will yield the maximum cellular stimulation (compared tocells with the ligand-dependent channel blocked, or not present) withsmallest coefficient of variation of the signal among the different testwells, at the lowest electric field strength, and at the lowest dutycycle for passage of current through the electrodes. After a particularset of parameters is chosen, a titration of staining concentrations forthe voltage sensor dye(s) should be performed as described above, tofurther optimize the signal size and coefficient of variation of theresponses. These procedures (dye concentrations, electric fieldstrength, and stimulus duration and frequency) can be iterated tofurther optimize the signal.

[0359] It should be possible to screen for both agonists andantagonists. By choosing stimulation parameters such that the responseis half-maximal, agonists will reduce the response while antagonistswill increase it. Better screening windows may be obtained bystimulating at higher (agonist assay) or lower (antagonist assay)frequencies.

[0360] During these experiments, the response will be compared for cellswith active channels, and for cells in which the channels arepharmacologically blocked. If a suitable pharmacological agent is notavailable, the blocked state can be emulated with an un-transfected cellline. The optimal stimulus parameters will yield the smallestcoefficient of variation of the difference in signals of the two cellpopulations. The present invention also includes methods for thequantitative determination of cellular and ion channel parameters, andfor the quantification of the pharmacological effects of test compoundson these parameters using electrical stimulation.

[0361] b. Quantitative Measurements of Membrane Resistances

[0362] After the electrical stimulus ends, the cell transmembranepotential relaxes to a new resting potential. In the case ofvoltage-dependent channel assays, the channels will generally close orinactivate, and the final resting equilibrium potential will be the sameas before the stimulus. In most cases, the charge built up on themembrane capacitance will dissipate exponentially through the membraneresistance. The membrane time constant is simply the product of themembrane capacitance and the membrane resistance, τ_(m)=R_(m)C_(m). Itcan be readily determined by measuring the membrane capacitance and themembrane time constant.

[0363] The average membrane capacitance for cells commonly used in theseassays is independent of the exogenous channel, and can easily bemeasured by patch clamp methods. The membrane time constant can bereadily measured by measuring the rate of decay of the transmembranepotential and fitting this data to an exponential decay function. Thusby dividing the membrane time constant by the average membranecapacitance for the given cell type, we can quantitatively determine theresting or leak membrane resistance.

[0364] A similar analysis can be made to quantitatively measure themembrane resistance while a voltage-dependent channel is open. Duringthe electrical stimulation, the transmembrane potential will also relaxapproximately exponentially towards a new equilibrium potential. Thus,the membrane time constant of the voltage change at the beginning of thestimulus constitutes a measurement of the time-averaged membraneresistance. Using appropriate scaling factors to account for thefraction of the time that the channel is actually open, we can make aquantitative estimate of the open-channel membrane resistance.

[0365] i. Measurement of Release from Inactivation Time Constant

[0366] Opening an inactivation ion channel requires holding thetransmembrane potential below a threshold for a time on the order ofseveral milliseconds. This release from inactivation has importantphysiological implications. For example, release from inactivationforces a refractory period which prevents back-propagation of actionpotentials, and limits the maximum firing rates of neurons.Pharmacological manipulation of this property may be therapeuticallyrelevant

[0367] Using repetitive electrical stimulation, we can estimate theaverage release from inactivation time. This can be done by usingelectric field pulses of variable width. When the pulse width fallsbelow the release from inactivation time, fewer channels will beactivated and the transmembrane potential rise in response to thestimulation will drop.

[0368] d. Measurement of the Open Channel Time

[0369] The open channel time τ_(open) is a function of the inactivationproperties of the channel. We can detect pharmacological manipulation ofthis parameter in a medium- to high-throughput mode by stimulating atvery high frequency. For example, consider an assay for avoltage-dependent sodium channel using the multiple stimulus method.With a fixed monophasic square wave stimulus kernel repeated at a steadyrate, the voltage response increases as the stimulus repetition rateincreases. This is because the sodium channel spends relatively moretime open at higher frequency. However, if the inter-pulse intervalbecomes shorter than the open channel time, the activated sodiumchannels will be driven negative, and thereby deactivated, by thesubsequent stimulus pulse. The stimulation burst frequency at which theresponse flattens is related to the open channel time.

[0370] e. Electrical Stimulation as an Extracellular Current ClampDevice

[0371] In whole-cell recording, current clamp is a mode in which commandcurrents can be driven into the cell while recording the transmembranepotential. Although patch-clamp recording is extremely precise, it is avery low-throughput technique. At an absolute maximum under perfectconditions, a highly trained scientist could determine cellularparameters at a rate of about ten cells per hour. Often, the level ofdetail obtained with the patch-clamp technique is not necessary for drugscreening, but there is currently no method for exchanging detail forspeed. High speed is absolutely crucial for screening large compoundlibraries.

[0372] The electric field stimulation techniques discussed herein permita new type of current-clamp electrophysiology which we callextracellular current clamp. Voltage-dependent channels can be used todrive command currents into cell cultures, allowing determination ofseveral cellular and channel properties. Extracellular current clamp hasa very high throughput, so that it will be possible to obtain highinformation content of the pharmacological effects of compound librariesagainst specific ion channel targets. The pharmacology and physiology ofa channel can be studied directly, or the channel can be used as acurrent generator for the study of the cell membrane itself or a secondion channel.

[0373] While the ultimate precision of the microscopic parametersobtainable with the extracellular current clamp cannot yet approach thepatch-clamp method, we now have the ability to exchange informationcontent for throughput. That is, the degree of precision at which tomake measurements can be arbitrarily set. With a single set of stimulusparameters, large libraries can be screened for potential interestingcompounds. A medium throughput secondary screen using a titration ofcompound concentrations can be performed on the hits to determinepotency and specificity. Finally, we can determine such therapeuticallyrelevant properties such as use-dependence and mechanism of action byvarying the stimulus parameters in the presence of the compounds. Atevery stage, the measurements are automatically averaged over manycells, greatly reducing uncertainties associated with cell-to-cellvariability.

[0374] There are at least two additional advantages of the extracellularcurrent clamp as compared to patch-clamp analysis. First, the integrityof the cell membrane is not altered during electric field stimulation.The intracellular fluid is completely replaced with pipette solutionduring whole-cell patch clamp recording. Many proteins within the cell,including ion channels, are extremely sensitive to modulators,intracellular messengers, and the ionic environment. The components ofthe cytoplasm are only crudely known, so the soluble components in theintracellular space are always altered. Therefore, the ‘normal’physiological state of the cell is only approximated during whole-cellpatch clamp analysis, but remains intact when using extracellularcurrent clamp.

[0375] Second, most cells experience dramatic alterations in geneexpression and behavior when in contact with other cells. Because mostcells also make gap junctional connections with neighboring cells,whole-cell patch clamp analysis is only reliable when cells arecompletely isolated from each other. Extracellular current clamp can beused on cells independently of their degree of confluence, so the cellsmay be more physiologically relevant. We can use extracellular currentclamp to find out if there are any effects of cell-cell contact onchannel electrophysiology. Then, in conjunction with gene expressionanalysis, we can relate these changes to regulatory components of thecell.

[0376] f. Electrical Stimulation as an Extracellular Voltage ClampDevice

[0377] In voltage-clamp, the transmembrane potential of the cell iscontrolled while monitoring the current flow. Voltage clamp is generallyachieved by adding a feedback loop to a current clamp circuit. In thecase of the whole-cell method, this can easily be achieved with the useof two pipettes simultaneously attached to the same cell. One pipettepasses a command current, while the other senses the voltage. A feedbackcircuit compares the measured voltage with the command voltage, andadjusts the command current accordingly. Generally, because the cellmembrane resistance is large compared to the access resistance of thepipette, the same pipette can be used to command the current and measurethe voltage. Compared to current clamp, voltage clamp is generally amore powerful method for electrophysiological analysis. Ion channels areextremely sensitive to transmembrane potential, so that analysis of datais far more straightforward when dealing with current measurements at afixed voltage.

[0378] Extracellular current clamp can be converted to a voltage clampmethod by adding a feedback loop between the voltage measurement (thefluorescence of the sensor dye) and the current generator (the stimulusparameters). In this case, a transmembrane potential dye with sufficientspeed is required. The dye combination CC2-DMPE/DiSBAC₆(3) has asubmillisecond time constant and should be sufficiently fast to captureall but the fastest cellular events. Based upon the difference of thecommand voltage and the transmembrane potential measurements, a computerwill alter the stimulus parameters. The stimulus parameters are relatedto the current driven into the cell, so we can determine the time courseof the current as a function of the command voltage. This method shouldprove useful in determining the mechanism of action of pharmacologicalagents upon ion channels targets.

[0379] g. Assays for Intracellular Compartments

[0380] The stimulation methods described herein can also be used tomodulate the transmembrane potentials of intracellular organelles thathave phospholipid membranes, including the mitochondria and the nucleus.This can be accomplished by first increasing the conductance of theplasma membrane either by electropermeablization or through the additionof ionophores such as valinomycin or gramicidin A. Then, theintracellular space is no longer insulated from the applied electricfield. This allows an electric field applied to the saline to generatetransmembrane potential changes across the membranes of intracellularorganelles. Then, by staining the cells with dyes which are sensitive tothe ion concentration or transmembrane potential, and which are targetedonly to the specific organelle membrane of interest, the methodspresented herein can be used to modulate and assay the ion channels ofthese organelles. Targeting can be achieved, for example via the use ofa naturally fluorescent protein containing suitable subcellular locationsignals as are known in the art.

[0381] 2. Introduction of Exogenous Molecules

[0382] Dielectric breakdown of mammalian cell membranes occurs if theelectric potential across the membrane exceeds about 200 mV (Teissie andRols, 1993, Biophys. J. 65:409-413). When the membrane breaks down,pores are formed through the membrane, bridging the intracellular andextracellular spaces. The number and size of the pores increases withincreasing transmembrane potentials (Kinoshita and Tsong, 1977, Nature268:438-441). Increasing the electric field strength above about 60 V/cmon typical mammalian cell lines can electropermeablize the cells. Atrelatively low fields, small pores are created in the cell membranewhich apparently are large enough to admit small ions, but not largeenough to admit molecules as large as DNA (Tsong, 1991, Biophys. J.60:297-306). These pores totally depolarize the cell, driving thetransmembrane potential to near zero. By electropermeablizing cells andmonitoring the transmembrane potential change with a voltage-sensitivedye, the present invention can be used to determine the restingtransmembrane potential of a cell. This will be useful for determiningpharmacological interactions with cells or ion channels, either as aprimary or a secondary screen. For example, in a compound screen againsta voltage-dependent sodium channel, one could perform a multiplestimulus protocol to determine channel activity. Then, by following witha permeablizing protocol, one could determine whether or not the cellmembrane had a normal resting potential in the presence of the compound.

[0383] Additionally, using a highly polarized cell line such as RBLcells, voltage sensitive dyes could be easily calibrated byelectropermeablization. The starting transmembrane potential undervarious conditions (for example, various concentrations of extracellularpotassium), and the final transmembrane potential afterelectropermeablization is zero. Additionally, the size of the porescreated by electropermeablization increases as a function of the appliedelectric field. Below 50 V/cm, no pores are created. Between about 60V/cm and 100 V/cm, pores large enough to admit monovalent ions arecreated. Above around 600 V/cm, pores large enough to admit DNA arecreated (Tsong, 1991, Biophys. J. 60:297-306). Thus, this invention canbe used to create pores of defined size in the cell membranes, in ahigh-throughput manner. This could be useful for many applications,including delivery to the intracellular space of impermeant ions,impermeant test compounds or other modulators, DNA or RNA for thepurpose of transient or stable transfection, and fluorescent or otherindicator dyes.

[0384] 3. Drug Discovery and Screening

[0385] a. Drug Screening

[0386] The present invention provides for the reliable detection of testcompounds that modulate ion channel function that is significantly moreversatile and robust than previous assay systems. Importantly, thepresent invention provides the ability to modulate the transmembranepotential in intact cells without the requirement of pharmacologicalagents, or membrane destruction, and loss of intracellular contents, asin patching clamping. By providing the ability to externally modulatethe transmembrane potential of living cells, the present inventionenables a wide variety of ion channels to be assayed.

[0387] Furthermore, this ability to modulate precisely the voltagedependent state of an ion channel, has important advantages for drugdiscovery where it provides the opportunity to screen for compounds thatinteract preferentially with one state, (i.e. use-dependent blockers).For example, several known therapeutically useful drugs (includinganti-arrhythmics, anti-convulsants, and local anesthetics) are known tofunction as use-dependent blockers of voltage-dependent sodium and/orcalcium channels. In each case, total blockade of the targeted channelwould typically result in death. Certain conditions, such as chronicpain, arrhythmia, and convulsions occur when cells become over-active.These conditions can be alleviated or eliminated by blocking thechannels if they begin to open too often. Compounds that are capable ofblocking the channel, but which bind preferentially to the activated orinactivated states(s) rather than the resting state(s), can reduce theexcitability of muscle and neurons. These drugs are effective becausethey do not affect the channel under normal circumstances, but block itonly when necessary to prevent hyper-excitability. However existingmethods of analysis that are compatible with high throughput screeningdo not provide the ability to routinely control the activation state ofthe ion channel in real time.

[0388] In particular, the present invention provides for a method forscreening the effect of a test compound on an ion channel in a definedfunctional state within a cell. The method involves modulating thetransmembrane potential of the cell via the use of repetitive electricalstimulation to cycle the ion channel of interest through its activationcycle and to set the transmembrane potential to a desired level suitablefor a specific activation state, or transition between states. Then,during or after this process a test compound is added to the cell, andthe transmembrane potential is measured.

[0389] Typically the results obtained in the presence of the testcompound will be compared to a control sample incubated in the absenceof the test compound. Control measurements are usually performed with asample containing all components and under the same stimulationconditions, as for the test sample except for the putative drug.Additional control studies can be carried out with the ion channel inanother voltage dependent state to specifically identify state specifictest compounds. Detection of a change in transmembrane potential in thepresence of the test agent relative to the control indicates that thetest agent is active and specific on the ion channel in that state, orduring the transition from one state to another.

[0390] Transmembrane potentials can be also be determined in thepresence or absence of a pharmacologic agent of known activity (i.e., astandard agent) or putative activity (i.e., a test agent). A differencein transmembrane potentials as detected by the methods disclosed hereinallows one to compare the activity of the test agent to that of thestandard agent. It will be recognized that many combinations andpermutations of drug screening protocols are known to one of skill inthe art and they may be readily adapted to use with the presentinventions disclosed herein to identify compounds, which affect ionchannels and or transmembrane potentials. Use of the present inventionsin combination with all such methods are contemplated by this invention.

[0391] In another aspect the present invention includes the use of asecond ion channel in conjunction with electrical stimulation methodsdescribed herein to set the resting, or stimulated transmembranepotential to a predefined value thereby providing for the ability toassay a first ion channel of interest. In one embodiment the second ionchannel is a voltage regulated sodium or calcium channel which enablesthe generation of sustained positive transmembrane potentials. Inanother embodiment the second ion channel is a voltage regulatedpotassium channel, enabling the generation of negative transmembranepotentials. The use of these second ion channels enables the electricalstimulation method to be used to set the transmembrane potential tovirtually any predefined level.

[0392] Because this assay format involves two ion channels, modulatorsof either channel will affect the voltage response. In this caseadditional control studies may be carried out with the parental cellline expressing only the second ion channel used to set thetransmembrane potential. Compounds that block the first ion channel canthen be re-tested separately to find out if they have activity againstthe second ion channel.

[0393] Typically the test compounds screened will be present inlibraries of related or diverse compounds. The library can haveindividual members that are tested individually or in combination, orthe library can be a combination of individual members. Such librariescan have at least two members, preferably greater than about 100 membersor greater than about 1,000 members, more preferably greater than about10,000 members, and most preferably greater than about 100,000 or1,000,000 members.

[0394] i. Selectivity and Toxicology of Candidate Modulators

[0395] Once identified, candidate modulators can be evaluated forselectivity and toxicological effects using known methods (see, Lu,Basic Toxicology, Fundamentals, Target Organs, and Risk Assessment,Hemisphere Publishing Corp., Washington (1985); U.S. Pat. No. 5,196,313to Culbreth (issued Mar. 23, 1993) and U.S. Pat. No. 5,567,952 to Benet(issued Oct. 22, 1996).

[0396] For example primary cell lines, or tissue slices can be used toscreen for the effect of the candidate modulator on the response of theion channel of interest in its native physiological context. Forexample, to screen for drugs that exhibit specific, and/or selectiveeffects on heart cells it may be preferable to use myocytes or other invitro cell culture model cell lines. In this case, a primary screencould be completed in a myocyte derived cell line to identify compoundsthat either shorten, prolong or block electrically-induced actionpotentials.

[0397] The secondary screen would then be designed to identify compoundsthat exhibit potentially adverse effects on the body. For example, thiscan be accomplished by screening for the effects of the candidate drugon electrically excitable tissues such as heart or neuronal tissues, orimmortalized cell cultures derived from these tissues. These tissuesplay critical roles within an organism and any undesired effect of thecandidate drug on the ability of these tissues to be electricallystimulated would be predicted to create potential serious side effectswhen administered. As a consequence, active compounds that also impairedthe ability of these tissues to function could be eliminated fromconsideration as a drug candidate at an early stage, or have medicinalchemistry performed to reduce the side effects.

[0398] Additional toxicological analysis of candidate modulators can beestablished by determining in vitro toxicity towards a cell line, suchas a mammalian (preferably human) cell line. Candidate modulators can betreated with, for example, tissue extracts, such as preparations ofliver, such as microsomal preparations, to determine increased ordecreased toxicological properties of the chemical after beingmetabolized by a whole organism, or via their ability to be degraded viaCytochrome P450 systems as described in commonly owned U.S. patentapplication Ser. No. 09/301,525, filed Apr. 28, 1999, U.S. patentapplication Ser. No. 09/301,395 filed Apr. 28, 1999 and U.S. applicationSer. No. 09/458,927 filed Dec. 10, 1999. The results of these types ofstudies are often predictive of toxicological properties of chemicals inanimals, such as mammals, including humans.

[0399] The toxicological activity can be measured using reporter genesthat are activated during toxicological activity or by cell lysis (seeWO 98/13353, published Apr. 2, 1998). Preferred reporter genes produce afluorescent or luminescent translational product (such as, for example,a Green Fluorescent Protein (see, for example, U.S. Pat. No. 5,625,048to Tsien et al., issued Apr. 29, 1998; U.S. Pat. No. 5,777,079 to Tsienet al., issued Jul. 7, 1998; WO 96/23810 to Tsien, published Aug. 8,1996; WO 97/28261, published Aug. 7, 1997; PCT/US97/12410, filed Jul.16, 1997; PCT/US97/14595, filed Aug. 15, 1997)) or a translationalproduct that can produce a fluorescent or luminescent product (such as,for example, beta-lactamase (see, for example, U.S. Pat. No. 5,741,657to Tsien, issued Apr. 21, 1998, and WO 96/30540, published Oct. 3,1996)), such as an enzymatic degradation product. Cell lysis can bedetected in the present invention as a reduction in a fluorescencesignal from at least one photon-producing agent within a cell in thepresence of at least one photon reducing agent. Such toxicologicaldeterminations can be made using prokaryotic or eukaryotic cells,optionally using toxicological profiling, such as described inPCT/US94/00583, filed Jan. 21, 1994 (WO 94/17208), German Patent No69406772.5-08, issued Nov. 25, 1997; EPC 0680517, issued Nov. 12, 1994;U.S. Pat. No. 5,589,337, issued Dec. 31, 1996; EPO 651825, issued Jan.14, 1998; and U.S. Pat. No. 5,585,232, issued Dec. 17, 1996).

[0400] Alternatively, or in addition to these in vitro studies, thebioavailability and toxicological properties of a candidate modulator inan animal model, such as mice, rats, rabbits, or monkeys, can bedetermined using established methods (see, Lu, supra (1985); andCreasey, Drug Disposition in Humans, The Basis of Clinical Pharmacology,Oxford University Press, Oxford (1979), Osweiler, Toxicology, Williamsand Wilkins, Baltimore, Md. (1995), Yang, Toxicology of ChemicalMixtures; Case Studies, Mechanisms, and Novel Approaches, AcademicPress, Inc., San Diego, Calif. (1994), Burrell et al., Toxicology of theImmune System; A Human Approach, Van Nostrand Reinhold, Co. (1997),Niesink et al., Toxicology; Principles and Applications, CRC Press, BocaRaton, Fla. (1996)). Depending on the toxicity, target organ, tissue,locus, and presumptive mechanism of the candidate modulator, the skilledartisan would not be burdened to determine appropriate doses, LD₅₀values, routes of administration, and regimes that would be appropriateto determine the toxicological properties of the candidate modulator. Inaddition to animal models, human clinical trials can be performedfollowing established procedures, such as those set forth by the UnitedStates Food and Drug Administration (USFDA) or equivalents of othergovernments. These toxicity studies provide the basis for determiningthe therapeutic utility of a candidate modulator in vivo.

[0401] ii. Efficacy of Candidate Modulators

[0402] Efficacy of a candidate modulator can be established usingseveral art-recognized methods, such as in vitro methods, animal models,or human clinical trials (see, Creasey, supra (1979)). Recognized invitro models exist for several diseases or conditions. For example, theability of a chemical to extend the life-span of HIV-infected cells invitro is recognized as an acceptable model to identify chemicalsexpected to be efficacious to treat HIV infection or AIDS (see, Dalugeet al., Antimicro. Agents Chemother. 41:1082-1093 (1995)). Furthermore,the ability of cyclosporin A (CsA) to prevent proliferation of T-cellsin vitro has been established as an acceptable model to identifychemicals expected to be efficacious as immunosuppressants (see,Suthanthiran et al., supra, (1996)). For nearly every class oftherapeutic, disease, or condition, an acceptable in vitro or animalmodel is available. Such models exist, for example, forgastro-intestinal disorders, cancers, cardiology, neurobiology, andimmunology. In addition, these in vitro methods can use tissue extracts,such as preparations of liver, such as microsomal preparations, toprovide a reliable indication of the effects of metabolism on thecandidate modulator. Similarly, acceptable animal models may be used toestablish efficacy of chemicals to treat various diseases or conditions.For example, the rabbit knee is an accepted model for testing chemicalsfor efficacy in treating arthritis (see, Shaw and Lacy, J. Bone JointSurg. (Br) 55:197-205 (1973)). Hydrocortisone, which is approved for usein humans to treat arthritis, is efficacious in this model whichconfirms the validity of this model (see, McDonough, Phys. Ther.62:835-839 (1982)). When choosing an appropriate model to determineefficacy of a candidate modulator, the skilled artisan can be guided bythe state of the art to choose an appropriate model, dose, and route ofadministration, regime, and endpoint and as such would not be undulyburdened.

[0403] In addition to animal models, human clinical trials can be usedto determine the efficacy of a candidate modulator in humans. The USFDA,or equivalent governmental agencies, have established procedures forsuch studies (see, www.fda.gov).

[0404] iii. Selectivity of Candidate Modulators

[0405] The in vitro and in vivo methods described above also establishthe selectivity of a candidate modulator. The present invention providesa rapid method of determining the specificity of the candidatemodulator. For example, cell lines containing related ion channel familymembers can be used to rapidly profile the selectivity of a testchemical with respect both to its ability to inhibit related ionchannels, and their relative ability to modulate different voltagedependent states of the ion channels. Such a system provides for thefirst time the ability to rapidly profile large numbers of testchemicals in order to systematically evaluate in a simple, miniaturizedhigh throughput format the ion channel selectivity of a candidatemodulator.

[0406] iv. An Identified Chemical, Modulator, or Therapeutic andCompositions

[0407] The invention includes compositions, such as novel chemicals, andtherapeutics identified by at least one method of the present inventionas having activity by the operation of methods, systems or componentsdescribed herein. Novel chemicals, as used herein, do not includechemicals already publicly known in the art as of the filing date ofthis application. Typically, a chemical would be identified as havingactivity from using the invention and then its structure revealed from aproprietary database of chemical structures or determined usinganalytical techniques such as mass spectroscopy.

[0408] One embodiment of the invention is a chemical with usefulactivity, comprising a chemical identified by the method describedabove. Such compositions include small organic molecules, nucleic acids,peptides and other molecules readily synthesized by techniques availablein the art and developed in the future. For example, the followingcombinatorial compounds are suitable for screening: peptoids (PCTPublication No. WO 91/19735, 26 Dec. 1991), encoded peptides (PCTPublication No. WO 93/20242, 14 Oct. 1993), random bio-oligomers (PCTPublication WO 92/00091, 9 Jan. 1992), benzodiazepines (U.S. Pat. No.5,288,514), diversomeres such as hydantoins, benzodiazepines anddipeptides (Hobbs DeWitt, S. et al., Proc. Nat. Acad. Sci. USA 90:6909-6913 (1993)), vinylogous polypeptides (Hagihara et al., J. Amer.Chem. Soc. 114: 6568 (1992)), nonpeptidal peptidomimetics with aBeta-D-Glucose scaffolding (Hirschmann, R. et al., J. Amer. Chem. Soc.114: 9217-9218 (1992)), analogous organic syntheses of small compoundlibraries (Chen, C. et al., J. Amer. Chem. Soc. 116:2661 (1994)),oligocarbamates (Cho, C. Y. et al., Science 261: 1303 (1993)), and/orpeptidyl phosphonates (Campbell, D. A. et al., J. Org. Chem. 59: 658(1994)). See, generally, Gordon, E. M. et al., J. Med Chem. 37: 1385(1994). The contents of all of the aforementioned publications areincorporated herein by reference.

[0409] The present invention also encompasses the identifiedcompositions in a pharmaceutical composition comprising apharmaceutically acceptable carrier prepared for storage and subsequentadministration, which have a pharmaceutically effective amount of theproducts disclosed above in a pharmaceutically acceptable carrier ordiluent. Acceptable carriers or diluents for therapeutic use are wellknown in the pharmaceutical art, and are described, for example, inRemington's Pharmaceutical Sciences, Mack Publishing Co. (A. R. Gennaroedit. 1985). Preservatives, stabilizers, dyes and even flavoring agentsmay be provided in the pharmaceutical composition. For example, sodiumbenzoate, acsorbic acid and esters of p-hydroxybenzoic acid may be addedas preservatives. In addition, antioxidants and suspending agents may beused.

[0410] The compositions of the present invention may be formulated andused as tablets, capsules or elixirs for oral administration;suppositories for rectal administration; sterile solutions, suspensionsfor injectable administration; and the like. Injectables can be preparedin conventional forms, either as liquid solutions or suspensions, solidforms suitable for solution or suspension in liquid prior to injection,or as emulsions. Suitable excipients are, for example, water, saline,dextrose, mannitol, lactose, lecithin, albumin, sodium glutamate,cysteine hydrochloride, and the like. In addition, if desired, theinjectable pharmaceutical compositions may contain minor amounts ofnontoxic auxiliary substances, such as wetting agents, pH bufferingagents, and the like. If desired, absorption enhancing preparations(e.g., liposomes), may be utilized.

[0411] The pharmaceutically effective amount of the composition requiredas a dose will depend on the route of administration, the type of animalbeing treated, and the physical characteristics of the specific animalunder consideration. The dose can be tailored to achieve a desiredeffect, but will depend on such factors as weight, diet, concurrentmedication and other factors which those skilled in the medical artswill recognize. In practicing the methods of the invention, the productsor compositions can be used alone or in combination with one another, orin combination with other therapeutic or diagnostic agents. Theseproducts can be utilized in vivo, ordinarily in a mammal, preferably ina human, or in vitro. In employing them in vivo, the products orcompositions can be administered to the mammal in a variety of ways,including parenterally, intravenously, subcutaneously, intramuscularly,colonically, rectally, nasally or intraperitoneally, employing a varietyof dosage forms. Such methods may also be applied to testing chemicalactivity in vivo.

[0412] As will be readily apparent to one skilled in the art, the usefulin vivo dosage to be administered and the particular mode ofadministration will vary depending upon the age, weight and mammalianspecies treated, the particular compounds employed, and the specific usefor which these compounds are employed. The determination of effectivedosage levels, that is the dosage levels necessary to achieve thedesired result, can be accomplished by one skilled in the art usingroutine pharmacological methods. Typically, human clinical applicationsof products are commenced at lower dosage levels, with dosage levelbeing increased until the desired effect is achieved. Alternatively,acceptable in vitro studies can be used to establish useful doses androutes of administration of the compositions identified by the presentmethods using established pharmacological methods.

[0413] In non-human animal studies, applications of potential productsare commenced at higher dosage levels, with dosage being decreased untilthe desired effect is no longer achieved or adverse side effectsdisappear. The dosage for the products of the present invention canrange broadly depending upon the desired affects and the therapeuticindication. Typically, dosages may be between about 10 μg/kg and 100mg/kg body weight, and preferably between about 100 μg/kg and 10 mg/kgbody weight. Administration is preferably oral on a daily basis.

[0414] The exact formulation, route of administration and dosage can bechosen by the individual physician in view of the patient's condition.(See e.g., Fingl et al., in The Pharmacological Basis of Therapeutics,1975). It should be noted that the attending physician would know how toand when to terminate, interrupt, or adjust administration due totoxicity, or to organ dysfunctions. Conversely, the attending physicianwould also know to adjust treatment to higher levels if the clinicalresponse were not adequate (precluding toxicity). The magnitude of anadministrated dose in the management of the disorder of interest willvary with the severity of the condition to be treated and to the routeof administration. The severity of the condition may, for example, beevaluated, in part, by standard prognostic evaluation methods. Further,the dose and perhaps dose frequency, will also vary according to theage, body weight, and response of the individual patient. A programcomparable to that discussed above may be used in veterinary medicine.

[0415] Depending on the specific conditions being treated, such agentsmay be formulated and administered systemically or locally. Techniquesfor formulation and administration may be found in Remington'sPharmaceutical Sciences, 18th Ed., Mack Publishing Co., Easton, Pa.(1990). Suitable routes may include oral, rectal, transdermal, vaginal,transmucosal, or intestinal administration; parenteral delivery,including intramuscular, subcutaneous, intramedullary injections, aswell as intrathecal, direct intraventricular, intravenous,intraperitoneal, intranasal, or intraocular injections.

[0416] For injection, the agents of the invention may be formulated inaqueous solutions, preferably in physiologically compatible buffers suchas Hanks' solution, Ringer's solution, or physiological saline buffer.For such transmucosal administration, penetrants appropriate to thebarrier to be permeated are used in the formulation. Such penetrants aregenerally known in the art. Use of pharmaceutically acceptable carriersto formulate the compounds herein disclosed for the practice of theinvention into dosages suitable for systemic administration is withinthe scope of the invention. With proper choice of carrier and suitablemanufacturing practice, the compositions of the present invention, inparticular, those formulated as solutions, may be administeredparenterally, such as by intravenous injection. The compounds can beformulated readily using pharmaceutically acceptable carriers well knownin the art into dosages suitable for oral administration. Such carriersenable the compounds of the invention to be formulated as tablets,pills, capsules, liquids, gels, syrups, slurries, suspensions and thelike, for oral ingestion by a patient to be treated.

[0417] Agents intended to be administered intracellularly may beadministered using techniques well known to those of ordinary skill inthe art. For example, such agents may be encapsulated into liposomes,then administered as described above. All molecules present in anaqueous solution at the time of liposome formation are incorporated intothe aqueous interior. The liposomal contents are both protected from theexternal micro-environment and, because liposomes fuse with cellmembranes, are efficiently delivered into the cell cytoplasm.Additionally, due to their hydrophobicity, small organic molecules maybe directly administered intracellularly.

[0418] Pharmaceutical compositions suitable for use in the presentinvention include compositions wherein the active ingredients arecontained in an effective amount to achieve its intended purpose.Determination of the effective amounts is well within the capability ofthose skilled in the art, especially in light of the detailed disclosureprovided herein. In addition to the active ingredients, thesepharmaceutical compositions may contain suitable pharmaceuticallyacceptable carriers comprising excipients and auxiliaries whichfacilitate processing of the active compounds into preparations whichcan be used pharmaceutically. The preparations formulated for oraladministration may be in the form of tablets, dragees, capsules, orsolutions. The pharmaceutical compositions of the present invention maybe manufactured in a manner that is itself known, for example, by meansof conventional mixing, dissolving, granulating, dragee-making,levitating, emulsifying, encapsulating, entrapping, or lyophilizingprocesses. Pharmaceutical formulations for parenteral administrationinclude aqueous solutions of the active compounds in water-soluble form.Additionally, suspensions of the active compounds may be prepared asappropriate oily injection suspensions. Suitable lipophilic solvents orvehicles include fatty oils such as sesame oil, or synthetic fatty acidesters, such as ethyl oleate or triglycerides, or liposomes. Aqueousinjection suspensions may contain substances that increase the viscosityof the suspension, such as sodium carboxymethyl cellulose, sorbitol, ordextran. Optionally, the suspension may also contain suitablestabilizers or agents that increase the solubility of the compounds toallow for the preparation of highly concentrated solutions.

[0419] Pharmaceutical preparations for oral use can be obtained bycombining the active compounds with solid excipient, optionally grindinga resulting mixture, and processing the mixture of granules, afteradding suitable auxiliaries, if desired, to obtain tablets or drageecores. Suitable excipients are, in particular, fillers such as sugars,including lactose, sucrose, mannitol, or sorbitol; cellulosepreparations such as, for example, maize starch, wheat starch, ricestarch, potato starch, gelatin, gum tragacanth, methyl cellulose,hydroxypropylmethyl-cellulose, sodium carboxymethylcellulose, and/orpolyvinylpyrrolidone (PVP). If desired, disintegrating agents may beadded, such as the cross-linked polyvinyl pyrrolidone, agar, or alginicacid or a salt thereof such as sodium alginate. Dragee cores areprovided with suitable coatings. For this purpose, concentrated sugarsolutions may be used, which may optionally contain gum arabic, talc,polyvinyl pyrrolidone, carbopol gel, polyethylene glycol, and/ortitanium dioxide, lacquer solutions, and suitable organic solvents orsolvent mixtures. Dyestuffs or pigments may be added to the tablets ordragee coatings for identification or to characterize differentcombinations of active compound doses. For this purpose, concentratedsugar solutions may be used, which may optionally contain gum arabic,talc, polyvinyl pyrrolidone, carbopol gel, polyethylene glycol, and/ortitanium dioxide, lacquer solutions, and suitable organic solvents orsolvent mixtures. Dyestuffs or pigments may be added to the tablets ordragee coatings for identification or to characterize differentcombinations of active compound doses. Such formulations can be madeusing methods known in the art (see, for example, U.S. Pat. No.5,733,888 (injectable compositions); U.S. Pat. No. 5,726,181 (poorlywater soluble compounds); U.S. Pat. No. 5,707,641 (therapeuticallyactive proteins or peptides); U.S. Pat. No. 5,667,809 (lipophilicagents); U.S. Pat. No. 5,576,012 (solubilizing polymeric agents); U.S.Pat. No. 5,707,615 (anti-viral formulations); U.S. Pat. No. 5,683,676(particulate medicaments); U.S. Pat. No. 5,654,286 (topicalformulations); U.S. Pat. No. 5,688,529 (oral suspensions); U.S. Pat. No.5,445,829 (extended release formulations); U.S. Pat. No. 5,653,987(liquid formulations); U.S. Pat. No. 5,641,515 (controlled releaseformulations) and U.S. Pat. No. 5,601,845 (spheroid formulations).

V. EXAMPLES

[0420] The invention may be better understood with reference to theaccompanying examples, which are intended for purposes of illustrationonly and should not be construed as in any sense limiting the scope ofthe invention as defined in the claims appended hereto.

1. Example 1 Analysis of the Electrical Field Uniformity of ParallelPlate Electrodes in Standard Round Wells

[0421] To analyze the effect of various electrode, and well designs, aseries of two-dimensional numerical simulations of the electric fieldswere produced using the software analysis package Quickfield™ 4.1,(Student's Version, Tera Analysis, http://www.tera-analysis.com). Thissoftware package creates coarse-grained mesh type electrical fieldintensity maps by solving Poisson's equation with a finite-elementanalysis method in two dimensions. For the purposes of this analysis,the fringing effects due to the gap between the bottom of the electrodeand the bottom of the well were ignored, and the voltage drops from theelectrodes to the saline were also assumed to be negligible. The spatialresolution of the modeling is approximately 0.5 mm.

[0422]FIG. 7A shows the results of the simulation using 4 mm wideparallel plate electrodes (710) with a 4 mm gap with a standardelectrical potential difference of 2V located in a standard circular96-well. In this figure, the outer circle (700) is the edge of the well,the two vertical lines (710) are the electrodes, and the dashed circlein the middle (720) is the area of observation. The gray area (740) isthe area in which the electric field remains within ±10% of the meanfield in the area of observation. In the white area (730), the field isless than 10% of the mean, and in the black area (750), the field ismore than 10% greater than the mean field. Within the area ofobservation, the standard deviation of the field strength is 2% of themean, and the difference between the maximum and minimum fields is 10%of the mean. Thus, this geometry satisfies the stated requirements forfield uniformity for use in the present invention.

2. Example 2 Analysis of the Electrical Field Uniformity of PinElectrodes in Standard Round Wells

[0423] To determine the predicted field uniformity for two 1.0 mmdiameter round pin electrodes placed in a standard 6.2 mm diameter well,separated by a distance of 4.0 mm, simulations were completed with thesame conditions and assumptions as described in Example 1.

[0424] In FIG. 7B, the outer solid circle (705) is the edge of the well,the two smaller circles (715) are the electrodes, and the dashed circlein the middle is the area of observation. The gray area (745) is thearea in which the electric field remains within ±10% of the mean fieldin the area of observation. In the white area (735), the field is lessthan 10% of the mean, and in the black area (755), the field is morethan 10% greater than the mean field. Within the area of observation(725), the standard deviation of the field strength is 15% of the mean,and the difference between the maximum and minimum fields is 87% of themean. Thus, this geometry does not create uniform electrical fields andas a consequence is not suitable for use with the present invention.

3. Example 3 Analysis of the Electrical Field Uniformity of ParallelPlate Electrodes in Square Wells

[0425]FIG. 8A shows a simulation of the field profile for two 6 mm wideparallel plate electrodes with a 4 mm gap in a 6.2 mm square well. Inthis figure, the outer square (800) is the edge of the well. The twovertical lines (810) are the electrodes. The dashed circle in the middle(820) is the area of observation. Of particular note is that theelectric field scale for FIG. 8 has been greatly amplified compared toFIG. 7 to provide contrast for the variations in electrical fieldintensity. The gray area (840) is the area in which the electric fieldremains within ±1% of the mean field in the area of observation. In thewhite area (830), the field is less than 1% of the mean. In thissimulation, at no point is the field more than 1% greater than the meanfield. Within the area of observation, the standard deviation of thefield strength is 0.02% of the mean, and the difference between themaximum and minimum fields is 0.12% of the mean. Thus, this geometrygreatly improves the field uniformity.

[0426] The results of the simulation indicate that the primary source offield non-uniformity in the parallel plate system shown in FIG. 7Aderives from the rounded walls of the well. In a standard well with acircular cross section, the current density will spread out and thencontract as it passes from one electrode to the other, and thisspreading generates non-uniformity. This can be corrected by usingmultiwell plates with square wells.

4. Example 4 Analysis of the Effect of the Addition of InsulatorElements to Mask Off Rounded Areas of the Wells

[0427]FIG. 8B shows a simulation of the field profile for two 4 mm wideparallel plate electrodes with a 4 mm gap in a 6.2 mm diameter roundwell using the standard conditions and analysis procedures as describedin Example 1. Insulators are attached to the electrodes to mask off therounded areas of the well between the electrodes, as shown in FIG. 9A.In FIG. 8B, the outer circle (802) is the edge of the well. The twovertical lines (812) are the electrodes. The dashed circle in the middle(822) is the area of observation. The cross-hatched areas (862) areinsulators attached to the electrodes. The gray area (842) is the areain which the electric field remains within ±1% of the mean field in thearea of observation. In the white area (832), the field is less than 1%of the mean. In this simulation, at no point is the field more than 1%greater than the mean field. Within the area of observation, thestandard deviation of the field strength is 0.2% of the mean, and thedifference between the maximum and minimum fields is 1.0% of the mean.Thus, this geometry greatly improves the field uniformity over the casewhere no insulator is used, but not as much as in the case of squarewells.

[0428] The results demonstrate that the field uniformity in standardround well plates can be greatly increased by filling the area outsideof the area defined by the electrodes with an insulating material. Inpractice inert plastics such as nylon, tetrafluoroethylene,polycarbonate, or any other non-porous polymer, or glass, could be usedas the insulator material, provided that they are relatively stable toaqueous solutions, easily fabricated and preferably non-fluorescent. Theinsulator would typically be attached to the electrode, and would notobscure any of the area defined by the electrodes.

5. Example 5 Analysis of the Effect of Satellite Electrodes on FieldUniformity

[0429] To test whether it is possible to compensate for the loss ofcurrent into the curved edge of the well via the use of satelliteelectrodes, simulations were carried out at a variety of electrodegeometries. FIG. 9B shows one possible embodiment of this concept, andFIG. 8C shows the electric field profile when this geometry is analyzedusing Quickfield™ as described in Example 1. In this example, two extrapairs of 0.7 mm wide parallel plate electrodes were placed with a 2 mmgap. These electrodes are outside of the area of observation, and aremaintained at half the potentials of their parent electrodes.

[0430] In FIG. 8C, the outer solid circle (804) is the edge of the well.The two long solid vertical lines (814) are the parent electrodes, andthe four shorter solid vertical lines (816) are the satelliteelectrodes. The dashed circle in the middle (824) is the area ofobservation. The gray area (844) is the area in which the electric fieldremains within ±1% of the mean field in the area of observation. In thewhite area (834), the field is less than 1% of the mean, and in theblack area (854), the field is more than 1% greater than the mean field.Within the area of observation, the standard deviation of the fieldstrength is 0.2% of the mean, and the difference between the maximum andminimum fields is 1.2% of the mean. Thus, this geometry greatly improvesthe field uniformity over the case where no insulator is used, but notas much as in the case of square wells. This example demonstrated theuse of four satellite electrodes in a specific configuration. By addingmore satellite electrodes outside of the area of observation, and byproperly assigning their potentials as a function of the potentialsapplied to the parent electrodes, the electric field uniformity can, inprinciple, be improved to arbitrary precision.

[0431] For example in a round well configuration, field uniformity inthe center area of observation can be improved by subdividing theparallel plate electrodes into several pieces separated by thininsulating dividers, as depicted in FIG. 9D. The potential applied toeach electrode (expressed as a fraction of the potential applied to thecentral most piece) can be individually tuned to maximize the fielduniformity in the area of observation.

[0432] This concept can be expanded to allow the use of non-paralleldipper electrodes, which have several vertical conducting stripes, eachof which is independently controlled.

6. Example 6 Analysis of the Effect of the Meniscus on Electrical FieldUniformity

[0433] The meniscus created by dipper electrodes when inserted into awell generates variations of saline depth of around 10% across the areaof observation. This in turn generates variations in the electric fieldof around 10% across the area of observation. These variations existeven if the electrode design is predicted to create perfect fielduniformity. Thus, eliminating the meniscus effect will improve theactual field uniformity. One possible method to accomplish this is toadd an insulating barrier between the electrodes. FIG. 9C depicts onesuch embodiment, wherein the insulating barrier is used to create a flattop surface for the liquid in the well. The bottom of this barrier, whenthe electrodes are inserted into the well, would be designed to sitapproximately 2.5 mm above the bottom of the well. Thus, the barrierwould be partially immersed in saline, and its bottom surface woulddefine the top of the conductive chamber to be flat and perpendicular tothe electrode surfaces. In this way, the electric field would not beperturbed by irregularities in the surfaces of the conductive volume.

7. Example 7 Fabrication of Dipper Electrode Electrical Stimulator

[0434] In one embodiment of the electrical stimulator the device iscomprised of a self-locating frame that positions the dipper electrodesinto the array of wells in a 96 well multiwell plate format (FIG. 1).FIG. 1 depicts the inserted position of the electrode array. In thisexample, the electrical stimulation device can be assembled from threefunctional parts. The first part is the positioning frame (40) thatlocates the device relative to the plate wells. This frame is made ofmetal and is a snug fit to the multiwell plate. This frame serves as thelocating base for the second functional part of the system, theretractor mechanism.

[0435] The retractor system consists of shoulder bolts (70) and returnsprings (not visible). The springs are wrapped around the shoulderbolts, and press against the positioning frame (40) and the bottom ofthe insulating cover (90). The return springs hold the electrodeassembly in the retracted position until the electrodes are lowered intothe plate wells. The retractor mechanism locates the third functionalpart of the system, the electrode array.

[0436] The third functional part of the system is the electrode array.The electrode array is made up of eight pairs of identical electrodecombs (10). The electrode combs are made of stainless steel and areprecision laser cut to avoid distortion. Each comb has eight tabs ofsufficient width to nearly span the diameter of the multiwell platewells. The backbone of the comb forms the electrical connection to thetabs (50). Two of these combs form the electrode pairs that are insertedinto a column of eight wells. The combs are held in position relative toeach other by an insulating precision drilled plate (30) that locatesthe electrodes relative to the positioning frame. Insulating spacers(20) maintain electrode separation and index the combs to the drilledplate by means of a pinned interface. A second set of spacers (25)ensures precise positioning of the electrodes (10) relative to the plate(30). Alignment shafts (15) are inserted through alignment holes in thespacers (20) and the electrode combs (10) for additional stability. Thecombs and spacers are held in place against the drilled plate by aninsulating cover (90).

[0437] The device may be used manually by placing the device on themultiwell plate and pressing down on the electrode assembly to lower theelectrodes into the wells. When the electrodes are fully extended, apair of locking bars (60) is inserted to keep the electrodes extendedinto the wells. Alternatively the electrode array can be automaticallyinserted and retracted in to the wells via standard mechanical orrobotic control systems known in the art.

[0438]FIG. 3 shows a block diagram of the major electrical and opticalcomponents. Electrical stimuli were created via a high-power amplifier(320), driven by a pair of digital function generators (380 and 310). Inone embodiment the switch (330) was a National Instruments (Austin Tex.)ER-16 controlled by a National Instruments PC-DIO 24 digitalinput/output card on board the VIPR™ reader controller computer (300).The switch (330) allowed defined wells within a 96-well plate to beelectrically stimulated with any given time protocol. In this case, asingle column of eight wells was stimulated simultaneously. Theamplifier (320) was built using the APEX PA93 chip (Apex MicrotechnologyCorp, Tucson, Ariz.) following a circuit provided by the manufacturer.The amplifier had the following specifications: ±100V DC in, 100 GΩinput impedance, 20× voltage gain, ±90V out, ±3 A out, 10 Ω outputimpedance. The function generators were Tektronix (Beaverton, Oreg.)model AFG310. The first function generator (380) was triggered by theVIPR™ reader software when the stimulus pulse train was required tobegin, and produced a train of TTL pulses to trigger the second functiongenerator (310). The second function generator was programmed with thestimulus waveform kernel.

8. Example 8 Voltage Dependence of Electrical Stimulation

[0439] Wild type Chinese hamster ovary cells (CHO cells) endogenouslyexpress a voltage-dependent sodium channel and can be conveniently usedto validate and optimize electrical stimulation parameters. Besides thissodium channel, these cells appear to have gap-junctional connectionsbetween adjacent cells and a very small (˜20 pA) voltage-dependentoutward current.

[0440] The voltage dependent sodium channel in these cells (hereafterreferred to as NaV1) has electrophysiological characteristics similar torat brain type IIa sodium channels. Analysis of the current/voltagecharacteristics of this channel via standard electrophysiology revealsthat typical wild type CHO cells have an average peak current of 100 pAper cell at −20 mV. This corresponds to a membrane resistivity (R_(Na))of about 800 MΩ. Assuming a single-channel conductance of 10 pS, thissuggests that there are only ˜125 sodium channels per cell. In ourhands, CHO cells typically exhibit a resting transmembrane potential(R_(m)) of about −35 mV, a resting membrane resistance >10 GΩ, and acell membrane capacitance (C_(m)) of 15 pF.

[0441] To test the voltage dependency of electrical stimulation, wildtype CHO cells were seeded into 96 well microtiter plates and incubatedin growth medium for 24-48 hours. They were then rinsed with bathsolution 1 and stained for 30 minutes each with 10 μM CC2-DMPE(coumarin), then 3 μM DiSBAC₂(3) (ethyl oxonol as described in AppendixA1). A stimulator assembly with 96 pairs of stainless steel electrodes(4 mm wide, 4 mm gap) was placed atop the assay plate, as described inExample 7. The electrodes were lowered into the saline covering thecells and remained 0.5 mm from the bottom of the well. Ratiometricfluorescence measurements were made during electrical stimulation usinga VIPR™ reader as described above, and the data were analyzed accordingto the procedures in Appendix A2. At any one time, only one column ofeight wells was assayed; the remaining wells received no excitationlight or electrical stimulation. After each plate was assayed, theelectrodes were thoroughly rinsed with distilled water and dried withcompressed air, to prevent cross-contamination between plates.

[0442] To determine the transmembrane potential changes occurring in thecells as a result of electrical stimulation, multiwell plates containingthe cells were analyzed in a VIPR™ reader. The cells in a 3 mm diameterarea of observation located midway between the electrodes were excitedwith light at 400±7.5 nm. The light was generated by a 300 W xenon arclamp, and passed through a pair of a pair of dielectric interferenceband-pass filters to select the correct excitation wavelength. Light wasdirected to and from the cells via a trifurcated fiber optic cable, withone cable for excitation light and two for fluorescence emission.Simultaneous measurements of blue (460±20 nm) and orange (580±30 nm)signals were recorded from each well at 50 Hz, digitized and stored on acomputer. Initial assays were 15 seconds long, and consisted of a 6second stimulation of repetitive (90 Hz repetition rate) biphasic (5ms/phase) square-wave stimulation beginning at 2 seconds at theelectrical amplitudes shown. For two seconds before and seven secondsafter the stimulation burst, no current passed through the electrodes.FIG. 10 shows the ratiometric responses at various field strengths up to32 V/cm. In this case the apparent rise time of the recorded response islimited by the response time of the DiSBAC₂(3) that has a response timeconstant of around 1 second. Below pulse amplitudes of 10 V/cm, noresponse is detectable. Above 20 V/cm, the response is robust andincreases only slightly as the voltage is further increased up to 32V/cm. As shown in FIG. 11, at higher voltages, the peak response(measured after about 5 seconds) shows only further small increases inresponse. The data in FIG. 11 can be fitted to a Boltzman function,which had a midpoint at 18.0 V/cm with a 2.0 V/cm width. The sharpnessof the onset and the flatness of the response at high fields arestrongly suggestive of a threshold phenomenon. The electric field atwhich the response is half maximal (18 V/cm) corresponds toapproximately ±30 mV deviations in transmembrane potential at theextreme edges of the cells, using formulas previously published(Equation 1, see also Tsong, 1991, Biophys. J. 60:297-306; and assumingan average diameter of the cells of 30 μm). It is thereforequantitatively consistent with the stimulation mechanism described abovefor voltage-gated sodium channels normally in the inactivated state.

[0443] High intensity electrical fields can result in electroporation ofthe cell membranes resulting in large relatively non-specific changes intransmembrane potential (Tsong, 1991, Biophys. J. 60:297-306). Toestablish whether or not this is also a major factor in the responses ofthe cells to lower electrical field intensities used here, experimentswere conducted with the sodium channel specific toxin tetrodotoxin(TTX). If the effects of electrical stimulation can be blocked by thetoxin, this would suggest that the effect of electrical stimulation isprimarily mediated by the activation of sodium channels. The results ofthis experiment are shown in FIG. 12. The data was obtained withelectrical field strength of 33 V/cm and demonstrate that TTX was ableto completely block the effect of electrical stimulation with typicalpharmacological characteristics consistent with the blockage of sodiumchannels. The EC₅₀ from the fit to this data is 9 nM, similar to thereported value for TTX in rat brain type IIa (8 nM, West et al., 1992,Neuron 8: 59-70). The fact that this signal is blocked by TTX withnormal pharmacology is strong evidence that the signal generated viaelectrical stimulation is almost entirely due to NaV1.

9. Example 9 Variation of Cellular Response to Changes in Stimulus PulseWidth and Frequency

[0444] To examine the behavior of the cellular response as the stimuluspulse width and frequency were varied, experiments were carried outusing wild type CHO cells as described in Example 8 above at a constantfield strength of 25 V/cm, while varying the pulse duration andfrequency.

[0445] The results are displayed in FIG. 13. Each data point representsthe average of eight wells stimulated at the same time from experimentsderived from five separate plates of wild-type CHO cells. The resultsshow generally that as the frequency of stimulation increases themagnitude of the response increases. One would predict that this effectshould eventually saturate as the transmembrane potential is driven tothe sodium reversal potential (V_(Na)). In this case this does not occurbecause the sodium channel density is too low.

[0446] Increasing the pulse duration results in higher relative degreesof electrical stimulation at lower stimulation frequencies up to about10 ms, beyond which further increases are less pronounced. Very smallpulse durations (less than 1 ms) also limit the response, apparentlybecause the channels are not effectively released from inactivation. Toefficiently induce large cellular responses, the best stimulationparameters are typically in the range in which the pulse duration isgreater than, or equal to the time constant for recovery forinactivation, and sufficiently short so that the frequency ofstimulation is greater than the membrane time constant. Additionally theoptimal frequency of stimulation is typically less than the reciprocalof the average channel open time.

[0447] These experiments demonstrate that electrical stimulation can besuccessfully used even in cells that express even relatively low levelsof voltage dependent channels, and can be successfully completed underconditions that do not lead to significant electroporation or celldeath. These experiments also demonstrate methods by which stimuluspulse duration and repetition frequency can be optimized to produceresponses of a desired size.

10. Example 10 Analysis of CHO Cells Expressing an Exogenous SodiumChannel

[0448] Chinese hamster ovary cells were stably transfected with aplasmid encoding a voltage dependent sodium channel (hereinafterreferred to as NaV2) as described in section VI. Whole-cell patch clampanalysis was used to confirm the electrophysiological andpharmacological properties of this channel prior to analysis viaelectrical stimulation. The peak transient sodium current at −20 mV wasmeasured to be 600±300 pA (N=5), with an average cell membranecapacitance of 15±5 pF. The resting cell membrane resistance was toolarge to measure accurately (R_(L)>10 GΩ). The resting transmembranepotential was −31±3 mV.

[0449] To determine the threshold electric field for stimulation, cellsstably expressing the sodium channel were plated in 96-well plates andstained according to the protocol in Appendix A1. The electricalstimulation protocol involved a 20 Hz, 3 second burst of biphasic (5ms/phase) stimuli with variable field strength using the electricalstimulator described in Example 7.

[0450]FIG. 14 shows representative time traces at various fieldstrengths (each curve is the average of eight wells). At low fieldstrengths, there is no detectable cellular response, suggesting that theaverage transmembrane potential changes less than about 1 mV. Between 35and 90 V/cm, the response is stereotyped, with a fixed shape andamplitude. Above 90 V/cm, the peak response stays relatively constant,but the response decay time after the stimulus is removed becomesconsiderably extended.

[0451] Consistent with the experiments shown in Example 8, the responseinduced by electrical field strengths up to 85 V/cm could be inhibitedby TTX whereas the response obtained from cells stimulated above 90 V/cmcould not (data not shown). Therefore we conclude that the fast responseis due to the sodium-channel-opening mechanism outlined above, while theslow response is mainly caused by electropermeablization of the membraneby the electrical field.

[0452] This effect is more easily seen by comparing the behavior of thefast response (4 seconds after stimulation) and the slow response (tenseconds after stimulation) with increasing field strength. This data isshown in FIG. 15. Fitting the fast response to a Boltzman function, themidpoint of the early response was at E₅₀=26 V/cm, with a width ofΔE=3.5 V/cm. The response was independent of field strength between 40and 80 V/cm, with a slight increase when electropermeablization sets inabove 90 V/cm.

[0453] The slower response due to permeablization was first detectableat 90 V/cm, and is itself of potential use in some applications. Forexample, permeablization can be used for resetting the transmembranepotential to zero, or if the permeablization is selective for a specificion, for resetting the transmembrane potential to the equilibrium valuefor that ion. This could be useful, for example, in an assay for achannel that sets the transmembrane potential. Examples includepotassium and chloride leak channels, potassium inward rectifiers, andlow-voltage activated voltage-gated potassium channels.

[0454] These results are consistent with published studies in whichelectropermeablization begins with a threshold transmembrane potentialof around ±200 mV, independent of cell type (Teissie and Rols, 1993,Biophys. J. 65:409-413). Based on formulae reported in that article andwidely accepted in the literature, CHO cells with an average diameter of30 μm will experience ±200 mV transmembrane potential changes whenexposed to a 90 V/cm extracellular electric field.

11. Example 11 Determination of the Effective Release from InactivationTime and the Effective Open-Channel Sodium Conductance

[0455] To make quantitative estimates of the effective release frominactivation time and open Channel conductance, but without being boundto any specific mechanism of action, the following theory was developedfor experimental verification.

[0456] After opening, the sodium channels inactivate with avoltage-dependent time constant of order 1 millisecond. Because thecurrent passed by the open sodium channels is strongly voltage- andtime-dependent, it is not possible to easily generate an analyticalexpression for the voltage change after a single stimulation. However bymaking some simplifying approximations, we can model average idealizedresponses to create a testable theory. For the purposes here, we assumethat upon opening, the sodium channels behave as a linear conductanceabove V_(t)=−40 mV with a reversal potential at E_(Na)=+60 mV. Theconductance g_(Na) is determined as the maximal current obtained at −20mV in a whole-cell patch clamp experiment. The time dependence of thesodium channel conductance is simplified by assuming that, when thechannel activates, it has a fixed conductance g_(Na)=1/R_(Na) for afixed time τ_(Na)=1.0 ms, after which the channel inactivates.

[0457] Using a biphasic square wave stimulus kernel (each phase has atime t₁ and is repeated at a frequencyƒ=1/T), the total current enteringthe cell during T is: $\begin{matrix}\begin{matrix}{I = {C_{m}\frac{V}{t}}} \\{= \frac{q_{N\quad a} - q_{L}}{T}} \\{= {{\frac{\tau_{N\quad a}}{T\quad R_{N\quad a}}\left( {V_{N\quad a} - V} \right)\left( {1 - {\exp \left\lbrack {- \frac{t_{1}}{\tau_{r}}} \right\rbrack}} \right)} + {\frac{1}{R_{L}}{\left( {V_{L} - V} \right).}}}}\end{matrix} & (2)\end{matrix}$

[0458] Here, τ_(Na) is the time the sodium channels are open.R_(Na)=1/g_(Na) is the membrane resistance when the sodium channels areopen. R_(L) is the normal (leak) membrane resistance. V_(L) is the leakreversal potential (i.e. the resting membrane potential). V_(Na) is thesodium reversal potential. τ_(r) is the time constant for recovery frominactivation; this is actually a function of the hyperpolarizing voltageachieved during the pulse, but here we assume it to be a constant.

[0459] In reality, sodium channels from different parts of the cellexperience different membrane potential changes, and the parametersτ_(Na), τ_(r), and R_(Na) have strong dependence upon membranepotential. The full model would take into account the cell morphology, arandom distribution of cell orientations, and the potential and timedependence of these parameters. It would then be possible to convolutethese dependencies to produce effective values for these parameters.These procedures are too involved for the present discussion. We willinstead recognize that the values that are extracted from fits to theseequations represent complicated averages of the underlying channelproperties.

[0460] Solving equation (2) for the transmembrane potential changeduring stimulation (V−V_(L)) yields: $\begin{matrix}\begin{matrix}{{\left( {V - V_{L}} \right) = {\left( {V_{N\quad a} - V_{L}} \right){\frac{f}{f_{0} + f}\left\lbrack {1 - {\exp \left( {- \frac{t_{1}}{\tau_{rise}}} \right)}} \right\rbrack}}},{where}} \\{f_{0} = {\frac{R_{Na}}{R_{L}{\tau_{Na}\left( {1 - {\exp \left\lbrack {- \frac{t_{1}}{\tau_{r}}} \right\rbrack}} \right)}}\quad {and}}} \\{\tau_{rise} = \left( {\frac{1}{R_{L}C_{m}} + \frac{\tau_{N\quad a}f}{R_{Na}C_{m}}} \right)^{- 1}}\end{matrix} & (3)\end{matrix}$

[0461] If the stimulation is carried out for a long enough time suchthat a new transmembrane potential is reached, the steady-state equationis: $\begin{matrix}{\left( {V - V_{L}} \right) = {\left( {V_{N\quad a} - V_{L}} \right){\frac{f}{f_{0} + f}.}}} & (4)\end{matrix}$

[0462] To determine the effective release from inactivation time andopen channel conductance, experiments were conducted as described inexample 8, using a biphasic square wave kernel at a constant amplitudeof 43 V/cm at varying frequencies and with pulse durations of 20 ms, 10ms, 5 ms, 2 ms and 0.3 ms. The results, shown in FIG. 16, display theresponse as a function of stimulation frequency for several pulsedurations. In this case as predicted, the response saturates at highfrequencies as the transmembrane potential apparently approaches thesodium reversal potential. To determine the effective release frominactivation time and channel open time the response R was fitted to themodified Hill equation below. $\begin{matrix}{R = {1 + \frac{A\quad f}{f + f_{0}}}} & (5)\end{matrix}$

[0463] Equation (5) can be derived from equation (4) by recognizing thatthe ratiometric response R=1 for no transmembrane potential change, andis linear in the transmembrane potential change with an uncalibratedproportionality constant A.

[0464] In equation (5), A and ƒ₀ are adjustable parameters. The fittingwas performed using a non-linear least-squares analysis using Origin 6.0software (Microcal, Northampton Mass.).

[0465] The parameters T₀=1/ƒ₀ from equation (5) above were extractedfrom these fits and plotted against the pulse duration and are shown inFIG. 17. The line in this figure is a fit to an exponential decay, andfrom this fit, we extract the release from inactivation time constant(τ_(r))τ_(r)=5.7 ms and R_(L)τ_(Na)/R_(Na)=0.314.

[0466] Assuming that τ_(Na)=1 ms and R_(L)=45 GΩ, then R_(Na)=140 MΩ.This in turn means that the peak sodium conductance would be 100mV/140MΩ=700 pA . This is in excellent agreement with the value measuredin whole-cell patch clamp.

12. Example 12 Analysis of an Exogenous Sodium Channel in a Cell Linewith Other Endogenous Ion Channels

[0467] Wild-type HEK-293 cells typically express a variety of endogenouspotassium and chloride currents (Zhu et al., 1998, J. Neurosci. Meth.81:73-83), so that the resting membrane resistance for these cells is5-10 GΩ. As a consequence the membrane time constant for these cells iscorresponding smaller, thus for optimal stimulation of the cells, onewould predict that the electrical stimulation protocol should berepeated at relatively higher frequencies compared to cells withoutendogenous potassium channels in order to generate comparable signals.

[0468] To test that a voltage regulated sodium channel could beefficiently electrically stimulated using the present invention in thiscellular background, HEK-293 cells were stably transfected with avoltage dependent sodium channel hereinafter referred to as NaV3. Cellswere transfected and selected as described in section VI and labeledwith FRET dyes as described in Example 8. Cells were plated and loadedwith 15 μM CC2-DMPE and 2 μM DiSBAC₆ (3) and then subjected to a 25V/cm, biphasic stimulus train repeated at a frequency of 90 Hz and witha 5 ms/phase pulse duration. The stimulation pulse train occurred for atotal duration of 3 seconds and the digitization rate for datacollection was 50 Hz.

[0469] The response as a function of time (FIG. 18) shows a rapid (<20ms rise time) initial phase which decays with a time constant of about40 ms to a stable plateau. A small rebound potential change is alsopresent between the spike and the plateau. We interpret this behavior asdue to the activation of endogenous voltage-dependent potassium channels(K_(V)) that occur after the first stimulus pulse. Activation of theseendogenous potassium channels would be expected to cause a reduction oftransmembrane potential as potassium leaves the cell consistent with theexperimental data. As electrical stimulation continues the transmembranepotential reaches a new equilibrium which is set by the balance ofsodium influx into the cell and potassium efflux out of the cell. At theend of stimulation, the decay time constant of the response is about 143ms, corresponding to a leak resistance of about 9 GΩ.

[0470] To determine whether this overall smaller response could bereliably used for drug discovery were conducted to determine whether theeffects of TTX or tetracaine could be accurately characterized. Theresults shown in FIG. 19 demonstrate that the pharmacological inhibitionprofiles of these drugs using the present invention are consistent withthe known behavior of the NAV3 sodium channel with these agents. Thedose-response curve for TTX could be fitted with a Hill function with anEC₅₀=25 nM and Hill coefficient 1.1. The dose-response curve fortetracaine could be fitted to a curve with an EC₅₀=11 μM and Hillcoefficient 0.97. These results suggest that the response is caused bysodium channel activity and that pharmacological information on knownand unknown compounds can be obtained using this method.

13. Example 13 Analysis of HEK-293 Cells Expressing the NaV4 SodiumChannel

[0471] To determine whether the present method is generally applicableto a wide range of different sodium channels, HEK-293 cells were stablytransfected with another voltage dependent sodium channel, hereinafterreferred to as NaV4. These cells were transfected, selected and loadedwith FRET dyes as described in section VI and Example 8. The results ofa dose-response curve for tetracaine on this channel are shown in FIG.20. Here the data points are averages and standard deviations of eightwells and the solid line is a fit to a Hill function with an estimatedEC₅₀=35 μM and Hill coefficient 1.35. These results are consistent withthe known pharmacology of this ion channel and demonstrate again thatthe cellular response is caused primarily by sodium channel activity.

14. Example 14 Analysis of HEK-293 Cells Expressing a Mixture ofVoltage-Activated Chloride and Potassium Channels

[0472] A demonstration of the direct stimulation of voltage-dependentchloride and potassium channels was performed using wild-type HEK-293cells, which endogenously express a mixture of several voltage-activatedchloride and potassium channels (Zhu, Zhang et al. 1998). Wild-typecells were grown in 96-well microtiter plates and assayed at confluenceafter staining with the FRET dyes according to the protocol in AppendixA1. Initial stimulus parameters included a 3 second long electricalstimulation at 20 Hz with a biphasic square wave stimulus kernel with apulse duration of about 5 ms/phase. Stimuli were performed at varyingelectric field intensities to determine the threshold field strength fora measurable cellular response, and in the presence or absence ofpotassium channel blockers.

[0473]FIG. 21 shows the cellular voltage response obtained during thisexperiment. In this figure, each panel contains the ten-second timetrace of the response for a single well. The panels are laid out tomatch their relative positions on the plate. The vertical axis in eachpanel is the background subtracted, normalized fluorescence ratio of theFRET voltage sensitive dye combination CC2-DMPE/DiSBAC2(3), changes inthis quantity are roughly proportional to changes in the membranepotential. Each column had identical stimulation conditions, withincreasing electric field strength from left to right across the plate.The twelfth column of the 96 well plate (not shown) contained no cellsand were used for background subtraction. Rows 6-8 contained 10 mM TEAto block the voltage dependent potassium channels. At the lowest fieldstrengths tested, there was no detectable response. At intermediateelectrical fields, a negative voltage response can be seen which rapidlydecays when the stimulus is removed. At the highest fields a largepositive response is elicited. This behavior sets in above 50 V/cm,similar to the electropermeablization threshold seen in CHO cellsexpressing NaV1, (Example 8).

[0474]FIG. 22 shows the response averaged between 4.5 and 5.0 seconds ofstimulation as a function of the electric field intensity. The largepositive responses above 60 V/cm were excluded to show thechannel-dependent negative responses. The coefficient of variation ofthe response is generally extremely small, yielding exceptionally largescreening windows (see Appendix A3). For the unblocked data for 20-40V/cm, the difference between stimulated and unstimulated wells is over20 standard deviations.

[0475] Tetraethylammonium (TEA), a well-known potassium channel blocker(Hille, 1992, Ionic Channels of Excitable Membranes), was added to rows6, 7, and 8 at a fully-blocking concentration of 10 mM. This treatmentpartially blocks the response. This result is consistent with theexistence of both potassium (blocked by TEA) and chloride (unaffected byTEA) channels in these cells that respond to electrical stimulation. Theeffect of the potassium channels can be isolated by blocking thechloride channels with 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid(DIDS) or 4-acetamido-4-isothiocyanostilbene-2,2′-disulfonic acid (SITS;see Hille, 1992, Ionic Channels of Excitable Membranes). Then, the samecell line could be used to screen two channel classes.

15. Example 15 Identification of State Dependent Blockers

[0476] Any proposed screening system should preferably be able toreproduce the pharmacology of known compounds as determined by acceptedmethods. To verify that this was the case for the present invention, aseries of test compounds of defined activity were analyzed using a CHOcell line that expresses the NaV2 channel. To accomplish this, cellswere cultured in 96 well plates and stained with voltage sensitive dyesas described in Appendix A1. Test compounds were added to the cells withthe oxonol loading buffer. Unless otherwise noted, the compounds weretested as in replicates of 8, with ⅓ dilutions across eleven columns ofthe assay plate.

[0477]FIG. 23 shows the time traces for selected concentrations of thesodium channel blockers tetrodotoxin (TTX) and tetracaine.

[0478] Tetrodotoxin is a potent, reversible, non-state specific sodiumchannel antagonist. By comparison tetracaine is a use dependent sodiumchannel blocker that exhibits different affinities for different sodiumchannel states.

[0479] The results show that the present invention provides for highlyreproducible results with relatively little variability either betweensamples or between plates. In FIG. 23 the effect of TTX can be seen as aprogressive loss of response, without significant changes in the shapeof the response. By comparison with tetracaine the responses not onlydecreases, but changes shape as the concentration varies. The C.V. forthese experiments were 10% (TTX) and 9% (tetracaine), compared totypical CVs using the same voltage dyes, but traditional liquid additionwere 16% (TTX) and 18% (tetracaine).

[0480] Importantly the results also show that the present invention canidentify the state dependent blockage of the sodium channel bytetracaine. The use-dependent block of tetracaine is more apparent inthe dose-response curves shown in FIG. 24. For TTX, the channel block isindependent of the time window used for calculating the response. Fortetracaine, however, the blockade is an order of magnitude stronger at 3seconds than at 1 second. Under the same stimulation conditions, otheruse-dependent blockers (lidocaine and bupivicaine) showed a smalleramount of shift in the dose-response curves. The EC₅₀ values obtained bythe electrical stimulation protocol for lidocaine were similar to thehigh-frequency values reported in the literature (see Table 4); thissuggests that lidocaine and bupivacaine have fast enough use-dependenceto be fully saturated at the 20 Hz stimulus used here. This in turnsuggests that we can explore the use-dependent properties of localanesthetics by varying the stimulation frequency.

[0481] Table 4 lists the blocking concentrations for several sodiumchannel antagonists. The literature values reported have all beenmeasured using whole-cell patch clamping, and are thus based on directmeasurements of the sodium channel current. TABLE 4 Pharmacology of NaV2in the electrical stimulation assay b. Electric field Compoundstimulation Literature value Reference Tetracaine 0.19 Bupivacaine 1.0Lidocaine 30 11 a 97 d Phenytoin 24 19 a 36 d WIN-17317 0.009 0.009 b0.006 0.008 c saxitoxin 0.001 c verapamil 3 d capsaicin 1.6 amiloride>1000

[0482] In Table 4, the table entries are EC₅₀ values (in micromolar) forfits to the dose-response curves from each assay. Each experiment wasdone twice, with four wells per drug concentration per experiment. Ineach experiment, eleven concentrations were used, spanning five ordersof magnitude in concentration. Reported values are the averages of thecalculated EC₅₀ from each experiment. In the cases of use-dependentblockers, the lowest recorded values are quoted.

[0483] WIN-17317 and TTX are potent tonic blockers of a variety ofsodium channels. These compounds can be detected using the electricalstimulation format, which yields blocking potencies near the literaturevalues.

[0484] The first four drugs (lidocaine, bupivicaine, tetracaine, andphenytoin) are use-dependent blockers. That is, they have differentaffinities for the various states of the channel. They are of greattherapeutic relevance, since at the proper concentration, they can blockdamaging repetitive bursting of neurons and muscle cells while leavingnormal, low-frequency activity unaffected. In all cases, the measuredblocking concentration measured with electric stimulation is close tothe reported literature value. The electrical stimulation assay formatis the only reliable high-throughput method for detecting all modulatorsof sodium channels, including agonists, antagonists, and use-dependentblockers.

15A. Example 15A Further Identification of State Dependent Blockers

[0485] We further demonstrate the characterization of use-dependenteffect of several known channel blockers against the endogenous Na_(V)channels of Chinese Hamster Ovary cells (CHO) at a stimulation frequencyof 1 Hz. Our data demonstrate that the combination of electrical fieldstimulation and fast voltage-sensitive dyes provides an effectiveapproach to induce and measure use-dependent block. Moreover, thepotencies and use-dependent properties of test compounds determined bythis method are in accord with those reported for the same channel typeusing patch-clamp technique. This new technique permits rapid detectionand characterization of novel use-dependent compounds.

[0486]FIG. 32A shows data from four compounds used to illustrate theuse-dependent effect on fluorescence ratio response. The representativetraces of ratio response were elicited by repetitive electricalstimulation, delivered at 1 Hz for 20 seconds, in the presence of TTX,tetracaine, astemizole, and lidocaine, respectively. In theseexperiments, each 96-well cell plate was treated with two compounds,each of which had four replicates of titration from columns 1 to 10 tocover a broad range of concentrations, as indicated. Cells in column 11were used for control. The peak ratio response in the absence ofblockers typically exhibited a fast time-dependent decrease by 20-30% toa new steady-state level at the end of repetitive stimulation. The smallimpulse-like response seen at highest concentrations of compounds werepresumably due to transient membrane capacitive current induced byelectrical field. Use-dependent block of endogenous Na_(V)channels bytest compounds was characterized by decreased ratio of last peakresponse over first peak response at certain concentration range, asseen in the presence of tetracaine or astemizole. By contrast, onlyminor or no use-dependent block by lidocaine or TTX was observed,indicating that both blockers are relatively weak use-dependent againstendogenous Na_(V) channels at this frequency.

[0487]FIG. 32B superimposes traces of ratio response in the absence andpresence of 3.125 μM tetracaine and 6.25 nM TTX to demonstrate thedifference in development of block between two blocker types. As shownin the inset, tetracaine induced more additional block than TTX, eventhough the latter exhibited greater potency on inhibiting the firstresponse. These results clearly demonstrate that our screening systemcan readily identify use-dependent properties of compounds.

[0488]FIG. 33 summarizes the concentration dependence of fractionalblock of the first and the last peak ratio responses from data shown inFIG. 32A. The shift between two IC50s, measured respectively for the1^(st) and 20^(th) peak responses, gives an estimate of use-dependenceof a given compound. Here we use the ratio of IC50(1^(st) response) overIC50(20^(th) response), denoted as use-dependent index (UDI), to comparethe use-dependence of tested compounds. The 1^(st) and 20^(th) peakresponses were measured in the absence and presence of variousconcentrations of blockers. Data were plotted as the ratio of peakresponses in the presence of a given blocker to that in the absence ofthe blocker. Data points represent the mean±S.D. of 4 replicates. Theratio of IC50(pulse #1) over IC50(pulse #20) is proportional to theuse-dependence of a given blocker at 1 Hz. Titration was fit to aMichaelis-Menton equation: R/R₀=[blocker]^(n)/([blocker]^(n)+IC50^(n)),R₀ denotes the normalized response obtained in the absence of blockers,n is the slope of the fit curve, IC50 is the half-inhibitionconcentration of the blocker.

[0489] Table 5 shows use dependence results for eight tested compoundsand summarizes the best-fit values of concentration-responserelationships for those compounds. The rank of UDI istetracaine>yohimbine>amitriptyline>mexiletine>dibucaine>astemizole>lidocaine>TTX,suggesting that tetracaine and TTX were the most and least use-dependentcompounds at the 1 Hz stimulus frequency in our experiments. The IC50smeasured for the first peak ratio response are fairly comparable tothose obtained with patch-clamp techniques for the same channel type atmore negative holding potential. The latter condition is similar to ourexperimental condition where cells were stimulated from their restingmembrane potential (about −60 mV, data not shown). ^(a)TABLE 5Use-dependence of Na_(v) 1.2a blockers at 1 Hz ^(b)UDI IC50 (μM) Hillcoeff. IC50 (μM) Hill coeff. ^(c)AlogP Compound (1 Hz) @ 1^(st) peak @1^(st) peak @ 20^(th) peak @ 20^(th) peak (calculated) Published IC50(μM) tetracaine 8.40  2.77 ± 1.23 1.81  0.33 ± 0.20 0.66 2.96 yohimbine5.67 26.66 ± 7.26 2.05  4.70 ± 1.94 0.99 2.17 amitriptyline 3.24  5.19 ±1.57 2.65  1.60 ± 0.38 1.07 5.15 5.5 mexiletine 2.59 58.12 ± 11.01 3.0722.46 ± 5.31 0.98 2.33 ^(d)21 ˜ 533 dibucaine 2.36  1.19 ± 0.22 2.35 0.51 ± 0.11 1.19 3.53 astemizole 2.25  4.12 ± 2.03 2.67  1.83 ± 0.681.79 4.79

16. Example 16 Applicability for High Throughput Screening

[0490] For the purposes of high throughput screening, the responsesshould be reliable enough to confidently tell the difference betweenactive and inactive compounds. This can be quantified by examining thedistribution of the responses obtained under identical stimulationconditions, comparing native channels with fully blocked channels. Dueto experimental uncertainty and noise in the system, there will be somescatter in the responses. We would like to be able to statisticallyquantify this scatter, and use it to predict the probabilities ofmisidentifying responses. as either false positives or false negatives.

[0491] To do this a plate of cells expressing the NaV2 voltage-dependentsodium channel was loaded with the FRET dyes. One well per column was‘randomly’ spiked with 1 μM TTX, approximately 200 times thehalf-blocking concentration. The cells were assayed with a 20 Hz, 3 secburst of 25 V/cm, 5 ms/phase, biphasic stimuli. The results are shown inFIG. 25. The wells spiked with TTX can easily be distinguished by eye asthe wells with little or no detectable response.

[0492] The ratiometric response two seconds after the stimulus began isshown in FIG. 26. The two populations (blocked and unblocked) can easilybe distinguished. The average blocked response was 1.011±0.004 while theaverage unblocked response was 2.67±0.21. The coefficient of variationfor the unblocked response is 13%. The screening window (i.e. thedifference between the populations normalized to the standarddeviations, see Appendix A3) is 7.8(σ₁+σ₂), where σ₁=0.21 is thestandard deviation of the unblocked response and σ₂=0.004 is thestandard deviation of the blocked response. If we take the cutoff pointto distinguish blockers from nonblockers midway between the populations(at 1.042), then the rate of statistical false negatives and falsepositives (assuming a normal distribution) is 1−prob(7.75)=10⁻¹⁴. Thissuggests that during a screen of a large compound library (10⁸compounds), the probability of encountering a single false positive orfalse negative during the entire screen is only one in a million. Forcomparison, if the difference between the populations were only 3 andthe cutoff was optimally placed, the false positive/negative rate wouldbe 0.3%, a factor of 10¹¹ higher. For an actual screen, in which wewould want to include as hits compounds which do not give completeblock, a tradeoff exists between detecting weak pharmacological activityand the rate of false positives. If, for example, we desire a falsepositive rate of 0.1%, then in this screen we can put the screeningcutoff at 3.3 standard deviations below the mean of the unblockedresponse, or at 1.97. In this case, the rate of false negatives iseffectively zero, and compounds which block only 50% of the responsewill be identified as hits.

[0493] Mathematically, there are two reasons that the blocked andunblocked populations overlap so little. First, the coefficient ofvariation of the unblocked response is relatively small. That is, eachresponse is nearly identical to every other response. Second, andperhaps more importantly, there is absolutely no detectable responsefrom the blocked wells. The scatter from blocked wells is consequentlyextremely small, so that we can place the boundary for distinguishingthe populations very low.

[0494] In assays performed using liquid addition protocols forstimulation, addition artifacts generally give some small response withan associated scatter. The scatter of the blocked response reduces thescreening window, increases the probability of false positives and falsenegatives, and limits the screener's ability to identify partialblockers.

17. Example 17 Screening in Complex Cell Lines

[0495] The feasibility of electrical stimulation of cells expressingmultiple channels was demonstrated using cultures of the HL5 cell line.These cells were generated by immortalizing cardiac muscle cells(Claycomb et al., 1998, PNAS 95: 2979-84). They contain severalvoltage-activated sodium, calcium, and potassium channels, as well as astrong inward rectifier potassium current and potassium and chlorideleak currents. Cells were grown in 96-well microtiter plates and assayedat confluence. They were stained according to the protocol in AppendixA1. Ratiometric fluorescence measurements were made during electricalstimulation using VIPR™ as described above, and the data were analyzedaccording to the procedures in Appendix A2. Stimulus parameters werearbitrarily chosen to be: 3 second long burst at 10 Hz with a biphasicsquare wave stimulus kernel with a pulse duration of 5 ms/phase. Stimuliwere performed at varying electric fields to determine the thresholdfield. Two rows of wells contained 10 μM TTX to partially block thecardiac sodium channel, and two rows contained 10 mM TEA to block thevoltage-dependent potassium channels. FIG. 27 shows the normalizedresponses of each well. Generally as the electric field strengthincreases, the cellular response increases. The last three columns showsigns of electropermeablization as the voltage continues to increase. Incolumns 6, 7, and 8, the ratio actually rebounds below the startingratio, suggesting an after-hyperpolarization (a phenomenon caused byslow closing of voltage-dependent potassium channels).

[0496] The rate of the cellular response is extremely fast, and may beapparently limited by the ability of the ethyl oxonol to rapidlyredistribute within the membrane. The rapid response is consistent witha high resting conductance of the cell due to the leak currents and theexpression of potassium inward rectifier channels. TTX partially blocksthe positive response, indicating that it is at least partially due tothe voltage-dependent sodium current.

[0497]FIG. 28 shows the response of the untreated cells (rows 1-4) as afunction of the applied electric field. The response increasessigmoidally with the electric field. Above 50 V/cm, there is a sustainedsignal which is unaffected by TTX. As discussed previously, thisbehavior is consistent with the electropermeablization of the cellularmembrane at high electric field strengths. Also shown in FIG. 28 is thescreening window (see Appendix A3) as a function of the stimulus field.

[0498] These results demonstrate that HL5 cells can be effectivelyassayed using the electrical stimulation technique. Compounds which areknown to modify different ion channels cause detectable changes in theresponse. Because these ion channels are identical to those expressed bythe heart, such an assay would be useful as a secondary screen, toeliminate or mark for modification those compounds which may interferewith normal heart function. It could also be useful as a primary screen,to discover compounds which may have desirable effects on any one (or acombination) of the heart ion channels.

18. Example 18 Electrical Stimulation of Cell Cultures Using SurfaceElectrodes

[0499] Surface mounted electrodes were prepared on glass coverslipscoated with chromium (as an adhesion layer) and gold (as a conductivelayer). The metallized coverslips were custom-built by Thin FilmDevices, Inc. (Anaheim Calif.). The coverslips were one inch square,0.17 mm thick Corning 7059 glass. Metallization was performed by vacuumsputtering deposition. The chromium layer was approximately 1000 Åthick, and served as an adhesion layer. The gold layer was approximately5000 Å thick, and served as a conductive layer. The resistivity of thedeposited metal was less than 0.1 Ω/square. A 4-mm gap was etchedthrough the metal by hand-masking the metal surface with achemically-resistant polymer (S1400-27, Shipley Co., Marlborough Mass.),then etching through the metal layers with five minutes in Gold EtchantTFA, followed by five minutes in Chromium Etchant TFD (Transene Co.,Danvers Mass.). The coverslips were attached to the bottoms of 96 wellplates with silicone elastomer (Sylgard 184 (Corning), cured 90 minutesat 70° C.). After sterilizing with 365 nm UV irradiation for 30 minutesand coating with the cell adhesion molecule poly-D-lysine (molecularweight 300,000, 1 mg/mL in Dulbecco's phosphate buffered saline for 30minutes, then rinsed 3 times with distilled water), living cells couldbe successfully grown and cultured on the electrode surfaces.

[0500] To validate the surface electrode stimulator CHO cells at aninitial density of approximately 1000 cells/mm² were plated into thewells of the 96 well plate and left to attach for approximately 16hours. These cells were transfected to express a potassium channel,which set the transmembrane potential to around −80 mV, and the NaV3sodium channel. After reaching confluence, the cells were loaded withthe voltage-sensitive FRET dye combination of CC2-DMPE and DiSBAC₂ (3)as described in Appendix A1. The metal surface electrodes were connectedto the output of a pulse generator, which in this case was anexponential-decay electroporator (Gene Pulser II, Bio-Rad Corp.,Hercules Calif.). Ratiometric fluorescence imaging was performed on aZeiss Axiovert TV microscope, equipped with a 75 W xenon arc lamp lightsource. Excitation light was filtered using a 405±10 nm dielectricinterference filter and a 445 DXCR dichroic mirror. Emission light wassplit with a second 525XR dichroic mirror, and measured with a pair ofHamamatsu HC124 photomultiplier tubes (PMTs). One PMT had a 475±40 nmdielectric interference filter in front of it to monitor the bluefluorescent signal. The second PMT had a 580±35 nm dielectricinterference filter in front of it to monitor the orange fluorescentsignal. The optical filters and dichroic mirrors were purchased fromChroma Technology Corp., Battleboro Vt. Ratiometric fluorescence imagingwas performed on fields containing approximately 100 cells. Correctionfor background fluorescence was performed by measuring the blue andorange signals in a field with no cells, then subtracting these from thesignals obtained from the cells. Then the ratiometric signal,proportional to the transmembrane potential changes, was calculated asdescribed in Appendix A2.

[0501] The stimulation protocol used single, monophasic electric fieldpulses of variable amplitude. The pulses were exponential-decaywaveforms with a 4.3 ms decay time constant. The amplitude at thebeginning of the pulse was varied from zero to 56 V/cm.

[0502] A typical voltage response for CHO cells expressing a potassiumchannel and the NaV3 sodium channel after a three separate 45 V/cmstimulation responses are shown in

[0503]FIG. 29 for the same field of cells, demonstrating repeatabilityof the response. The speed of the response in this case is limitedprimarily by the response time of the mobile hydrophobic dye, which forthe ethyl oxonol used is about 0.5 second.

[0504] The average ratiometric response of a population of cells grownin a 96 well multiwell plate stimulated with monophasic stimuli ofvarying field strengths is shown in FIG. 30. The points in this curveare the average peak response of 4 stimulations on the same culture. Asis to be expected from an action-potential-type response curve, there isno detectable response below about 18 V/cm. The threshold region isrelatively narrow. Between about 20 and 40 V/cm the response increaseswith increasing field strength. Above 40 V/cm the response plateaus.

19. Example 19 Analysis of Wild-Type RBL Cells Expressing IRK1

[0505] Rat basophilic leukemia (RBL) cells endogenously express thepotassium inward rectifier channel IRK1 (Wischmeyer et al, PflugersArch. 429:809-819, 1995). This channel selectively conducts potassiumions, with a highly non-linear conductance characteristic. Theconductance is nearly linear below the potassium reversal potentialV_(K), and rapidly drops to near zero beginning at about 10 mV positiveof V_(K). Cells expressing large amounts of inward rectifier channelstend to have resting transmembrane potentials within a few millivolts ofV_(K).

[0506] On the side of the cell where the transmembrane potential isdriven positive by an external electric field applied to cellsexpressing IRK1 and few other ion channels, the IRK1 channels willrapidly close and cease conducting. On the side of the cell where thetransmembrane potential is driven negative, the IRK1 channels will openand pass potassium current. If this side of the cell is drivensufficiently negative, so that the local transmembrane potential is morenegative than V_(K), a net inward potassium current will exist. Thiscurrent will cause a positive global transmembrane potential change.Because the IRK1 channel does not inactivate, this current should besustained for as long as the external field is applied.

[0507] Adherent RBL cells were seeded into 96-well plates and loadedwith FRET dyes as described in Appendix A1. Three rows of wellscontained 400 μM barium chloride to block the IRK1 channel. The plateswere analyzed using a VIPR™ reader while being electrically stimulatedwith a biphasic stimulus train repeated at a frequency of 50 Hz and witha 5 ms/phase pulse duration. The stimulation pulse train occurred for atotal duration of 5 seconds and the digitization rate for datacollection was 50 Hz. The applied electric field was fixed for eachcolumn of eight wells, and was varied from 7.2 to 72 V/cm. The data wereanalyzed according to the procedures in Appendix A2. The normalizedratio after three seconds of stimulation was calculated, averaged forthe two population of wells (with and without barium block), and plottedas a function of the applied field in FIG. 31. The error bars arestandard deviations of the responses. Open squares are the responseswithout barium block; solid circles are the responses with barium block.The data from the wells with barium block indicate that there is nodetectable voltage change during stimulation until the field reaches 80V/cm, at which point some electropermeablization may be occurring. Theunblocked wells show nearly linear response above a threshold at around20 V/cm. This example clearly shows that the present invention can beused to modulate the transmembrane potential in either positive ornegative directions, depending upon the stimulus parameters and theproperties of the ion channels expressed by the cell.

20. Example 20 Paired-Pulse Stimulation Protocol for Screening andBiophysical Characterization of Candidate Ion Channel Modulators

[0508] In many electrophysiological studies, paired-pulse experimentshave been used to characterize the recovery of voltage-gated ionchannels from voltage inactivation and from blockade by channelblockers. Typically, voltage-gated channels are activated by a series oftwo consecutive activating voltage steps separated by various interpulseintervals (or recovery time) ranging from a few milliseconds to tens ofseconds. The dissociation rate of blocker from inactivated channels thencan be estimated from the ratio of two activated peak currents for thesecond and first pulse step and recovery time t. This relationshipusually can be best fit with a single or two-exponential equation:

I _(Na) at 2nd pulse/I _(Na) at 1st pulse=A1 exp (−t/τ _(fast))+A2 exp(−t/τ _(slow))+A3,

[0509] where A1, A2, and A3 denotes constants; τ_(fast) and τ_(slow) arethe time constants for the fast and slow recovery.

[0510] Many clinically used ion channel modulators have been found toattenuate the recovery phases, which represents an important mechanismof their actions. Our novel method utilizing fast-response dye andelectrical field stimulation has provided a new approach to characterizesuch drug (or test compound)-channel protein interaction, yet at ahigher-throughput format over conventional patch-clamping techniques.The following parameters can be estimated from our method:

[0511] a. dissociation constant of test compound

[0512] b. compound's frequency-dependence of recovery from

[0513] tonic block

[0514] use-dependent block

[0515] c. compound's concentration-dependence of recovery

[0516]FIGS. 34 and 35 show application of a pair pulse protocol usingNaVs and known use-dependent NaV antagonists. FIG. 34A shows an exampleof a stimulation protocol to study recovery of channels from tonic blockand use-dependent block. Δt denotes the time interval between two pulsetrains. FIG. 34B shows concentration dependent recovery from tetracaineand lamotrigine block at 0.5 s train interval. Cells expressing Nav1.8were stimulated with two 10-Hz monophasic pulse trains, separated by a0.5 s resting period. Each pulse train consisted of 3 ms, 80Vp-p singlepulses. Recovery of channel from block was determined by the ratio oftwo first peak fluorescence responses induced by each of pulse trains(2nd peak/1st peak), of which 1 indicates full recovery. These datademonstrated that the recovery of channels from lamotrigine block wasnot significantly dependent on the concentrations of lamotrigine testedat 0.5 seconds Δt. By contrast, the recovery of channels from tetracaineblock exhibited strong dependence on the concentrations of tetracaine,shown in FIG. 35.

21. Example 21 Paired-Pulse Stimulation Protocol Using Varying TimeIntervals

[0517] The recovery of channels from block using different timeintervals Δt was also considered for lamotrigine and tetracaine. FIG.36A shows the results for lamotrigine block at 50 μM for different timeintervals while FIG. 36B shows the results for tetracaine block at 4 μMfor different time intervals.

[0518] The concentrations of lamotrigine and tetracaine were set atapproximately the IC50 values, determined from previous experiment, tobe 50 μM and 4 μM, respectively. The time interval between two 10-Hzpulse trains was varied in each column of plates from 0.1 s to 4 s, asindicated. In the presence of 50 μM lamotrigine, the ratio of two firstpeak fluorescence ratio responses were not significantly changed byincreasing Δt, indicative of fast recovery of channel from lamotrigineblock. By contrast, the recovery of channel from tetracaine block wasmuch slower, which requires as long as 4 s of resting period. Thissuggests that the off-rate of tetracaine binding is significantly slowerthan that of lamotrigine. As shown in FIG. 37, tetracaine at 4 μMrequires approximately 1000 ms or 1 s to recover from use-dependentblock. This is in contrast to lamotrigine which recovers very quickly.

22. Example 22 Stimulation of Synchronous Voltage Oscillations and DrugScreening Method for Use-Dependent Compounds that Dampen, Reduce, andModulate the Frequency of Action Potential Firing or Repetitive VoltageTransients

[0519]FIG. 38 shows reduction of firing frequency by bupivacaine andlidocaine in Aδ- or C-type neuron slice preparation. Use-dependent blockof TTX-resistant Nav channels by bupivacaine and lidocaine resulted inslower firing rate in neuron slice preparation (Scholz et al., 1998).This effect can be reproduced by optical hexyl oxonol assay innon-excitable cells with the use of high-frequency monophasic pulses.

[0520]FIG. 39 shows induced spontaneous firing of Nav1.8 expressingcells is slowed by lamotrigine and lidocaine. The representativefluorescence ratio responses from Nav1.8 cells, induced by 90 Hz 50V 1ms monophasic pulses, in the absence and presence of 15.6 μM lidocaineor 125 μM lamotrigine. Without the presence of Nav blockers,high-frequency monophasic stimulation induced spontaneous oscillation offluorescence response at frequency of 6-10 Hz. Addition of lidocaine orlamotrigine gradually increased the interspike interval (or decreasedfiring frequency), accompanied by the reduction of peak response. Thisbehavior is analogous to the use-dependent effect of these blockers onprimary neurons.

23. Example 23 Detection of Use-Dependent Blocker of CaV with Method

[0521] Assays can also be developed using fast membrane potentialsensors, such as DISBAC₆(3) and CC2-DMPE voltage-sensitive FRET probes,to detect use-dependent compounds for voltage-gated Ca targets. Thefollowing illustrative example is for an N-type calcium channel usinghardware compatible with 96 well plates.

[0522] Prepare a solution of 2× CC2DMPE and DISBAC₆(3) by Vortexingtogether 10 mM CC2DMPE (in DMSO) and 10 mM DISBAC₆(3) (in DMSO) with anequivalent volume of 10% pluronic (in DMSO). Vortex in required amountof bath. Each cell plate will require 5 ml of this solution. Wash platewith a plate washer leaving a residual volume of 50 μl. Add 50 μL of 2×CC2DMPE to wells. Resulting concentrations of dyes will be 4 μM CC2-DMPEand 1.25 μM DISBAC₆(3). Stain for 30 minutes in the dark at RT. The testcompounds are prepared at 2× concentration in extracellular ringersolution and added to wells containing stained cells. Stimulationparameters such as stimulation frequency, pulse duration, stimulationvoltage can be easily manipulated to elicit desired responses.

[0523]FIG. 40 illustrates an example of how a use-dependent compoundsuch as mibefradil can be detected using this hexyl assay. The change inthe response lineshape at 5 μM compound is indicative of use-dependentblock.

24. Example 24 Technique for Identifying Voltage-Dependent Compounds (ASubset of Use-Dependent Activity) Using Stimulation Instrumentation andOptical Methods in the Presence of Different External PotassiumConcentrations

[0524] We have developed a dorsal root ganglion (DRG) neuron assay onthe electrical stimulation with optical detection platform that allowsus to detect and quantify voltage dependent blockers of the sodiumchannels in these neurons. The DRG neurons were dissociated fromneonatal rat pups and stained with FRET voltage sensing dyes. Theneurons were then placed in the middle of the well on a 96 well plateand preincubated with varying concentrations of the test compound.Electrical stimulation was applied and the resulting response recordedby the plate reader. Dose response plots were then constructed for eachcompound tested. Increased levels of potassium were then added todepolarize the neurons and move more of the sodium channels into theinactivated state. Dose-response plots before and after addition ofelevated potassium showed a leftward shift in the relationship and acorresponding decrease in the IC₅₀. This estimates the degree of voltageor state dependence for these compounds. FIG. 41 shows data fromtetracaine and lamotrigine, two well known voltage dependent sodiumchannel blockers before and after addition of 16 mM potassium. As shown,the high potassium levels depolarize the cells. The compounds' block isvoltage dependent, a type of use-dependent activity. These compoundsblock with higher affinity at depolarized membrane potentials asindicated in the leftward shifted dose response curves. This exampleillustrates also the use of this method with primary neurons.

25. Example 25 L Type Calcium Channel Antagonist Assay Using ElectricalStimulation and FRET Voltage Sensing Dyes

[0525] The L-type calcium channel plays an important role in cardiaccontraction and vascular smooth muscle tone. Antagonists of this targethave demonstrated utility in hypertension and arrhythmia. Drugdevelopment compounds for other indications should avoid block of theL-type calcium channel to minimize cardiovascular side effects. We havedeveloped a 96 well plate assay for antagonists of the L-type calciumchannel that incorporates electrical stimulation in the presence ofDISBAC₂(3) and CC2-DMPE voltage-sensitive FRET probes.

[0526] The following protocol was used. After washing the plate with aplate washer, the cells were stained with 10 μM CC2-DMPE in the requiredbath solution in the dark at room temperature for 30 minutes. The platewas then washed and stained with 3 μM DISBAC₂(3), including 500 μMVABSC1 in the solution. Compounds were added in the second stainingsolution. The cells were assayed on the hardware platform using anoptimized electrical stimulation protocol. FIG. 42 shows block of theL-type calcium channel with the potent dihydropyridinesnifedipine andnimodipine. The IC₅₀ value is similar to that reported in theliterature.

[0527] The present invention expands the applicability of electricalstimulation to include non-excitable cells, by providing instrumentationand methods that enable effective stepwise control of membrane potentialwithout resulting in significant electroporation. The present inventionachieves this result via the use of highly uniform, repetitive pulses ofelectrical stimulation applied to the medium surrounding the cells. Theapplied electric fields typically do not directly alter the averagetransmembrane potential of the cell, but instead create symmetricpositive and negative transmembrane potential changes on the sides ofthe cell facing the cathode and the anode, respectively.

[0528] The approach exploits the ion selectivity and the non-lineargating and conductance characteristics of voltage-dependent ionchannels. The approach also exploits the fact that typical intact cellshave long time constants for decay of transmembrane potential changes.Even in those cases where the charge injected into the cell by a singlestimulus pulse is too small to be detected reliably, appropriatelyapplied multiple stimulus pulses can build large net transmembranepotential excursions. By varying the number, duration, and the shape andamplitude of the pulses, it is possible to artificially set, or changethe transmembrane potential of living cells in a fashion that is similarto patch clamping. Other channels, leak currents or transporters thatare not classically considered voltage-dependent, can also be assayed byinducing transmembrane potential changes using a second,voltage-dependent channel and detecting the current flow ortransmembrane potential changes as a result of activation of the targetchannel or transporter.

[0529] The present method is robust, compatible with optical detectionmethodologies and readily amendable to a wide range of potentialapplications including high throughput screening for use in drugdiscovery. In many assay formats direct electrical stimulation avoidsthe requirement for liquid addition, making the assay simpler. Complexmanipulations of the transmembrane potential can readily be accomplishedusing variations in the stimulation protocol. Thus, virtually anyvoltage-sensitive channel can be induced to open regardless of the stateof inactivation or voltage dependency. For high throughput drugdiscovery this relaxes the requirements for specialized cell types, andallows assays to be rapidly performed with readily available cell lines.

[0530] All publications and patent documents cited in this applicationare incorporated by reference in their entirety for all purposes to thesame extent as if each individual publication or patent document were soindividually denoted.

VI. Appendices

[0531] A1. Staining Protocol of Voltage FRET Dyes

[0532] 1. Reagents (a) Assay buffer #1 140 mM NaCl 4.5 mM KCl 2 mM CaCl₂1 mM MgCl₂ 10 mM HEPES 10 mM glucose   pH 7.40, 330 mOs/kg (b) Pluronicstock (1000X) 100 mg/mL pluronic 127 in dry DMSO (c) Oxonol stock(3333X) 10 mM DiSBAC₂(3) in dry DMSO (d) Coumarin stock (1000X) 10 mMCC2-DMPE in dry DMSO (e) ESS-CY4 stock (400X) 200 mM ESS-CY4 in water

[0533] 2. Loading and Assay Protocol

[0534] 1. Preparation of CC2-DMPE loading buffer. Normally for a 96-wellplate, 10 mL of staining solution will be prepared per plate.

[0535] i) Mix equal volumes (10 μL) of coumarin stock and pluronic stockin a tube.

[0536] ii) Add 10 mL Assay Buffer #1 to tube while gently vortexing.

[0537] Loading concentration: 10 μM CC2-DMPE and 0.1 μg/ml pluronic.

[0538] 2. Prepare oxonol loading buffer:

[0539] i) Mix equal volumes (3.3 μL) of oxonol stock and pluronic stockin a tube.

[0540] ii) Add 10 mL Assay Buffer #1 to tube while gently vortexing.

[0541] iii) Add 25 μL ESS-CY4 while vortexing.

[0542] Loading concentration: 3 μM DiSBAC₂(3), 0.2 μg/ml pluronic, and0.5 mM ESS-CY4.

[0543] iv) If required, combine test compounds with the loading bufferat this time.

[0544] 3. Rinse cells twice with Assay Buffer #1, removing all fluidfrom wells each time.

[0545] 4. Add 100 μL CC2-DMPE loading buffer to each well. Incubate 30minutes at room temperature, avoiding bright light.

[0546] 5. Rinse cells twice with Assay Buffer #1, removing all fluidfrom wells each time.

[0547] 6. Add 100 μL oxonol loading buffer to each well.

[0548] 7. Incubate for 30 minutes at room temperature avoiding brightlight. Use immediately.

[0549] A2. Analysis of VIPR™ Reader Data

[0550] Data were analyzed and reported as normalized ratios ofintensities measured in the 460 nm and 580 nm channels. The process ofcalculating these ratios was performed as follows. On all plates, column12 contained Assay Buffer #1 with the same DiSBAC2(3) and ESS-CY4concentrations as used in the cell plates, however no cells wereincluded in column 12. Intensity values at each wavelength were averagedin initial (before the stimulus) and final (during the stimulus)windows. These average values were subtracted from intensity valuesaveraged over the same time periods in all assay wells. The ratiosobtained from samples in the initial (Ri) and final windows (Rf) aredefined as: $\begin{matrix}{{R\quad i} = \frac{\left( {{{intensity}\quad 460\quad {nm}},{{initial} - {{background}\quad 460\quad {nm}}},{initial}} \right)}{\left( {{{intensity}\quad 580\quad {nm}},{{initial} - {{background}\quad 580\quad {nm}}},{initial}} \right)}} & \left( {{A2}{.1}} \right) \\{{R\quad f} = \frac{\left( {{{intensity}\quad 460\quad {nm}},{{final} - {{background}\quad 460\quad {nm}}},{final}} \right)}{\left( {{{intensity}\quad 580\quad {nm}},{{final} - {{background}\quad 580\quad {nm}}},{final}} \right)}} & \left( {{A2}{.2}} \right)\end{matrix}$

[0551] Final data are normalized to the starting ratio of each well andreported as Rf/Ri.

[0552] A3. Screening Window

[0553] The screening window W for a response is defined as follows. Datafrom multiple wells at identical stimulus conditions are required. Thecontrol wells can either be pharmacologically blocked or untransfectedcell stimulated with the full electric field. Alternatively, one mightuse transfected cells with no stimulus applied.

[0554] Responses from experimental and control wells are measured. Theaverage and standard deviations of the responses in the experimental(R±ΔR) and control (C±ΔC) wells are calculated. The screening window isdefined as the difference between experimental and control signalsnormalized to the sum of the standard deviations. $\begin{matrix}{W = \frac{R - C}{{\Delta \quad R} + {\Delta \quad C}}} & \left( {{A3}{.1}} \right)\end{matrix}$

[0555] A general rule of thumb for an acceptable screening window isW>3. This allows one to choose a cutoff line midway between control andexperimental responses which ensures a false negative/positive rate lessthan 1%. Assuming a normal distribution, the false positive/negativerate as a function of the screening window W is: $\begin{matrix}\begin{matrix}{P_{false} = {1 - {{prob}(W)}}} \\{= {1 - {\frac{1}{\sqrt{2\pi}}{\int_{- W}^{+ W}{{\exp \left( {- \frac{t^{2}}{2}} \right)}\quad {t}}}}}}\end{matrix} & \left( {{A3}{.2}} \right)\end{matrix}$

TABLE A3.1. The false positive/negative rate P(W) as a function of thescreening window W as defined in Equation A3.1. This calculation assumesthat the cutoff for identification of a hit is placed an equal number ofstandard deviations away from the positive and negative controlresponses. W P(W) 1 0.3173 2 0.0455 3 0.0027 4 6.334E−5 5 5.733E−7 61.973E−9 7 2.559E−12 8 1.221E−15 9   <1E−18 10   <1E−18

What is claimed is:
 1. A method of characterizing the biological activity of a candidate compound comprising: placing a population of cells into an area of observation in a sample well; exposing said population of cells to said compound; exposing said population of cells to electric fields to produce a controlled change in transmembrane potential of said population of cells; wherein said electric fields comprise a first pulse series and a second pulse series with a pause between the first pulse series and the second pulse series; and monitoring changes in the transmembrane potential of said population of cells during at least a portion of said first pulse series and a portion of said second pulse series.
 2. The method of claim 1 wherein monitoring comprises optically monitoring.
 3. The method of claim 2 wherein optically monitoring comprises detecting fluorescence emission of a FRET based voltage sensor from an area of observation containing said population of cells.
 4. The method of claim 1, further comprising comparing data gathered from said first pulse series with data gathered from said second pulse series.
 5. The method of claim 1, wherein the changes in the transmembrane potential are indicative of ion channel recovery from block by said compound.
 6. The method of claim 1 wherein said pause has a duration that is at least as long as twice the time interval between any two pulses in the first pulse series.
 7. A method of characterizing the biological activity of a candidate compound comprising: placing a population of cells into an area of observation in a sample well; exposing said population of cells to said compound; exposing said population of cells to electric fields to produce a controlled change in transmembrane potential of said population of cells; wherein said electric fields comprise a series of pulses; monitoring changes in the transmembrane potential of said population of cells; and comparing concentration dependence of transmembrane potential response to said electric fields at a first portion of said series of pulses with concentration dependence of transmembrane potential response to said electric fields at a second portion of said series of pulses.
 8. The method of claim 7 wherein monitoring comprises optically monitoring.
 9. The method of claim 8 wherein optically monitoring comprises detecting fluorescence emission of a FRET based voltage sensor from an area of observation containing said population of cells.
 10. The method of claim 7, wherein the changes in the transmembrane potential are indicative of use dependent block of an ion channel in said population of cells. 